An Overview of the Immune System

Immunology is the scientific study of the immune system and immune processes. Immunity is the ability of an animal to resist disease.The Latin term ‘ immunis’ means exempt, referring to the protection from foreign invaders.The immune system consists of a complex network of specialized organs, cells and secreted molecules that is widely dispersed throughout the body. The integrity of the immune system is crucial for the survival of every living organism.

The immune system displays both enormous diversity and extraordinary specificity. The cells of the immune system originate in our bone marrow.Three lines of lymphocytes – B-,T- Lymphocytes and natural killer cells are derived from lymphoid stem cells of the bone marrow. The organs of the immune system are called the lymphoid organs, which produce , store and process lymphocytes. The lymphoid organs are divided functionally into primary and secondary organs. The primary lymphoid organs ( bone marrow, thymus) are the sites of differentiation of lymphoid progenitors, while the secondary lymphoid organs ( lymph nodes, spleen,submucosal lymphoid tissues) are the sites from which immune responses are mounted. The lymphatic system is the site and source of most immune activity. Cells in the immune system secrete two types of proteins: antibodies and cytokines. B-lymphocytes mature in the bone marrow. T-lymphocytes mature in the thymus gland.

Study of Immunology

Immunology is a broad discipline that encompasses specialities as diverse as biochemistry, microbiology, pathology, medicine, molecular genetics, anatomy and physiology. Immunology is closely related to human health and wellbeing than any other discipline of biological science.Immunology is a highly complex and rapidly evolving field. The field of immunology had undergone a dramatic expansion during the late 19^th and early 20^th centuries.

Scope of Immunology

Immunologists study the tissues, cells, molecules involved in host defense mechanisms.They attempt to understand how the immune system develops, how the body defense itself against disease and what happens, when it all goes wrong.Immunologists are trying their best to identify how the immune system is coordinated. They have been developing effective clinical applications:
1. Infection and immunity
2. Cancer Immunology
3. Transplantation Immunology
4. Cell and Immunotherapy
5. Vaccine development and active immunization
6. Gene therapy

Functions of the Immune System

The immune system , like any organization, has members that performs different functions to accomplish a common goal.
1. It provides defenses against pathogens.
2. It removes dead or worn out cells like RBCs.
3. It identifies and destroys abnormal cancer cells.
4. It protects from autoimmune diseases.
5. It rejects tissue cells of foreign antigens.

Hierarchical Structure of the Immune system

The immune system has a multilayered architecture with defenses provided at many levels. The immune system is a complex network security system of chemicals, cells, tissues and organs that work together to protect the body. The immune system is crucial to human survival. The purpose of the immune system is to maintain homeostasis, which includes protecting the body from pathogens and toxins that could disrupt the homeostasis.

Goals of the immune system

• Classifying –ability to distinguish between body’s own cells-self and foreign cells-nonself.
• Identifying-ability to distinguish one pathogen from another.
• Switch on –ability to respond when a pathogen invades.
• Switch off – ability to stop when danger passes.
• Remembering – ability to remember previously encountered pathogens .

Mechanism of immune response

There are two critical steps in the immune response.The immune system specifically recognizes and selectively eliminates pathogens. The detection and elimination of pathogens depend upon the chemical bonding established between receptors on the surface of an immune cell and epitopes found on the surface of a pathogen. The strength of the bond between a receptor and an epitope is called the affinity. The complementary receptor-epitope binding mono specifically similar to lock-key mechanism activates a complex system of signaling that mediates the immune response. The immune recognition phase is critical in the normal functioning of the system and accomplished by three sets of antigen binding molecules: the T-cell antigen receptor (TCR), the class I and class II molecules of the Major Histocompatibility Complex (MHC) and the B-cell antigen receptor (BCR). The effector phase is mediated by a variety of cells and soluble factors. Rarely the immune response is defective. Failure in ‘self’ recognition can result in autoimmune diseases.

Components of the immune system

There are two types of immune systems: Natural or non-specific and adaptive or specific.The immune systems together provide an excellent defense against foreign invaders.
Innate immunity is inherited and does not change over an animal’s lifetime( genetic immunity ).It provides a rapid first line of defense.It consists of things as physical barriers, chemical barriers, some cellular defenses, inflammation, fever and molecular defenses.It does not have the characteristics of memory and specificity.It’s main aim is preventing the pathogen from entering the body or slowing the growth of infectious agents.

Adaptive immunity refers to the resistance against infectious disease an individual acquires during lifetime.The main characteristics of are exquisite antigenic specificity and the ability to ‘remember’ different types of antigens. The diversity of recognition mechanisms (e.g., T cell receptors, immunoglobulins) is dramatically increased through somatic mutation and recombination.The adaptive immune system mounts a response that is highly specific for a particular pathogen. This unique specificity is provided by the antibodies and lymphocytes .Adaptive immunity is not dependent on innate immunity.But both systems cooperate to produce a more effective, evolved and vast defense mechanism against infectious agents. The hallmarks of adaptive immunity are specificity ( antigen specific immunity ), immune memory and immune tolerance.

Adaptive immunity includes humoral immunity and cell mediated immunity. Humoral or antibody mediated immunity involves the production of antibodies by differentiated B cells called plasma cells. Cell mediated results from the formation of activated T cells.

• Antibody mediated reactions acts against bacteria and viruses in the body fluids.
• Cell mediated responses are effective against intracellular pathogens.
• Antibody mediated (humoral) immunity is regulated by B cells.
• Cell mediated immunity is controlled by T cells.

Cells of the immune system

T lymphocytes
T cells play a central role both in humoral immune responses and cell – mediated responses.T cells are lymphocytes, T lymphocytes which develop in the thymus. T cells kill pathogens produce molecules like cytokines that stop their growth.The cells with a CD4 marker are called helper T cells ( Th). The CD8 positive cells that develop are cytotoxic T cells ( Tc cells). T lymphocytes have been switching on various aspects of the immune response and then switching them off.

B lymphocytes

B lymphocytes start their life in the bone marrow.B lymphocytes mass produce proteins (antibodies) which bind to and kill microbes. Each B cell is capable of producing abtibody of only one given specificity.The development of millions of B lymphocytes expressing millions of specificities insures that the immune system has the potential to respond to millions of different foreign insults. Some B cells become ‘memory cells’ which produce the ‘secondary respone’.

     Macrophages

Macrophages engulf antigens , process them internally, then display parts of them on their surface together with some of their own proteins. They sensitizes the T cells to reorganize these antigens.

The chemical messengers of the Immune System

Immunoglubulins(antibodies)

The chemical information of immunoglobulin was provided by Tiselius and Kabat in the early 1940s. In 1950s, Porter and Edelman revealed the basic structure of immunoglobulin molecule. Antibodies are products of antigen- activated B- lymphocytes. They are the main effectors of humoral immunity. They bind antigens with high specificity and affinity.

Structure of Immunoglobulins

This glycoprotein is Y-shaped molecules and has two identical binding sites for its antigen, one on either arm of the Y. This protein is composed of four polypeptide chains (two identical heavy chains and two identical and smaller light chains) held together by disulfide bonds. The antibody molecule has two identical light chains (L chains), each containing about 200 amino acids and two identical heavy chains (H chains), each made up of 400 amino acids. Each chain is made up of several different domains. The antigen-binding site is formed where a heavy chain variable domain (VH) and a light chain variable domain (VL) come close together. These are the domains that differ most in their sequence and structure in different antibodies.

The variable (V) regions

The first 100 or so amino acids at the N-terminal of both H and L chains vary greatly from antibody to antibody. These are the variable (V) regions. The amino acid sequence variability in the V regions is especially pronounced in 3 hypervariable regions. Together they construct the antigen binding site against which the epitope fits. Only a few different amino acid sequences are found in the C-terminals of H and L chains. These are the constant (C) regions.
The constant (C) regions

The two different kinds of C regions for their L chains producing kappa (κ) L chains and lambda (λ) L chains. There are five different kinds of C regions for their H chains producing mu (µ) chains (the H chain of IgM antibodies), gamma (γ) chains (IgG), alpha (α) chains (IgA), delta (δ) chains (IgD) and epsilon (ε) chains (IgE).

Fc and Fab regions

The proteolytic enzyme papain breaks each Ig molecule into 3 fragments at the hinge region. The single crystallizable fragment (Fc region) includes part of the constant domain that occupies the stem. There are 2 antigen-binding fragments (Fab region), which include the entire light chain and variable and constant portions of the heavy chain.

Subclasses of human immunoglobulins

There are nine chemically distinct classes of human immunoglobulins, four kinds of IgG and two kinds of IgA, plus IgM, IgE, and IgD. Immunoglobulins G, D, and E are similar in appearance.

The classes of human immunoglobulins are  based on structure and function.

·         IgM -secreted during primary response

·         IgG -secreted during secondary response

·         IgD -receptors for antigens on B cells

·         IgA -found in external secretions

·         IgF -promotes histamine release

Defense mechanisms of antibodies

 1.Opsonization
2. MAC cytolysis
3. Antibody-dependent cellular cytotoxicity (ADCC) by NK Cells
4. Neutralization of exotoxins
5. Neutralization of viruses
6. Preventing bacterial adherence to host cells
7. Agglutination of microorganisms
8. Immobilization of bacteria and protozoans.

Cytokines

They are non- antibody proteins produced by cells of the immune system (including T cells, B cells, monocytes, and macrophages). Cytokines include a diverse group of interleukins, interferons, and growth factors. Cytokines are chemical switches that turn certain immune cell types on and off. One cytokine, interleukin 2 (IL-2), triggers the immune system to produce T cells. Cytokines also are being studied for their potential clinical benefit.

Kinds of cytokines

1. Monokines - produced by mononuclear phagocytes
2. Lymphokines - produced by activated T cells, primarily helper T cells
3. Interleukins - name given to many cytokines, abbreviated as IL and given a     number

Functions of cytokines

·         Activation of the Immune cells

·         Promotion of cell growth, meaning maturation, and/or division

·         Attraction of cells to the site of infection

·         Destruction infected or malignant cells

·         Stimulation of phagocytic activity.

Chemokines

The name chemokine is a contraction of chemotactic cytokines. These are a large family of substances (more than 50) produced by many different leukocytes and tissue cells. They recruit leukocytes to sites of infection. They play a role in lymphocyte trafficking.
Properties of chemokines
Chemokines are produced by cells involved in both natural and specific immunity. They mediate and regulate immune and inflammatory responses. The secretion is brief and limited. Many individual cytokines are produced by many cell types and act on many cell types (they are pleiotropic). In many cases cytokines have similar actions (they are redundant).

Tumor Necrosis Factor, TNF-gamma

It is produced by activated macrophages. It is the most important mediator of acute inflammation in response to Gram-negative bacteria and other infectious microbes. It mediates the recruitment of polymorphonuclear leukocytes (PMNs) and monocytes to the site of infection: acts on the hypothalamus to produce fever and promotes the production of acute phase proteins by the liver.

Interleukin-2

It is produced mainly by helper T cells (CD4+); less by cytoxic T cells (CD8+). It promotes T cell division and increases production of other cytokines. It has autocrine functions on T cell proliferation.

Interleukin-4

It is produced mainly by Th2 subpopulation of helper T cells (CD4+). It stimulates immunoglobulin class switching to the IgE isotype and development of Th2 cells from naive CD4+ T cells. It also promotes growth of differentiated Th2 cells.

Interleukin-5

It is produced mainly by the Th2 subpopulation of helper T cells (CD4+). It promotes growth and differentiation of eosinophils and activates mature eosinophils.

Interferons (IFN)

There are three groups of interferons: IFN-alpha , IFN-beta , IFN-gamma. Twenty variants of IFN-alpha are produced by leukocytes in response to viruses. IFN-beta is a single protein produced by fibroblasts and other cells in response to viruses. Both IFN-alpha and IFN-beta inhibit viral replication and increase expression of class I MHC on cells. IFN-gamma is produced by the Th1 subpopulation of helper T cells (CD4+), cytotoxic T cells (CD8+), and NK cells. IFN-gamma functions in both natural and specific immunity.

Transforming Growth Factor (TGF-beta)

It is an inhibitory cytokine produced by T cells, macrophages, and many other cell types. It inhibits proliferation and differentiation of T cells. It inhibits activation of macrophages. It acts on PMN and endothelial cells to block the effects of pro-inflammatory cytokines.

The complement system

The complement system consists of more than 35 soluble and cell-bound proteins ,12 of which are directly involved in the complement pathways. The proteins account for 5% of the serum globulin fraction . Most of these proteins circulate as zymogens , which are inactive until proteolytic cleavage . The complement proteins are synthesized mainly by hepatocytes ; however, significant amounts are also produced by Monocytes, macrophages, and epithelial cells in the gastrointestinal and Genitourinary tracts.

Secretory molecules of non-specific immunity

These include organic acids in skin secretions, thiocyanate in saliva, low molecular weight fatty acids in the lower bowel; bile acids and low molecular weight fatty acids in lower GI tract; transferrin, lactoferrin, lysozyme, interferons, fibronectin, complement, acute phase proteins, etc. in serum; Interferons and tumor necrosis factor (TNF) at the site of inflammation. Transferrin and lactoferrin deprive organisms of iron. Interferon inhibits viral replication and activates other cells which kill pathogens. Lysozyme, in serum and tears, breaks down the bacterial cell wall (peptidoglycan). Fibronectin coats (opsonizes) bacteria and promotes their rapid phagocytosis. Complement components and their products cause destruction of microorganism directly or with the help of phagocytic cells. Acute phase proteins (such as CRP) interact with the complement system proteins to combat infections. TNF-alpha suppresses viral replication and activates phagocytes.

The cells of the Immune System

The basic functional units of the immune system are diverse cells belonging to the lymphoid and reticuloendothelial system. The immunoreactive cells are lymphocytes, monocytes, macrophages, mast cells and a few granulocytes. Lymphocytes are mainly responsible for specific immune responses. Immune cells move through the body in blood and lymph. Immune cells have the capacity to react when appropriately stimulated by antigens.
The lymphocytes are classified into T and B lymphocytes. T lymphocytes are responsible for cell mediated immunity. B lymphocytes are responsible for the expression of humoral mediated immunity (antibody production).

Origin of  Immune reactive cells

1. The lymphoid lineage gives rise to T lymphocytes (T cells), B lymphocytes (B cells) and natural killer cells (NK cells)

2. The myeloid lineage gives rise to monocytes, macrophages, Langerhans cells, dendritic cells, megakaryocytes and granulocytes (eosinophils, neutrophils, basophils)

Types of leucocytes

Agranulocytes: They have a clear cytoplasm and also known as mononuclear leukocytes. They include monocytes (4%) and lymphocytes (24%). The monocytes transform into the tissue macrophages, which are active phagocytes, and have become resident in connective tissue
Granulocytes: They contain abundant granules in their cytoplasm. They are also known as polymorphonuclear leukocytes. (PMNs), because of their lobed nuclei. The granulocytes are identified as different types, based on their staining characteristics with certain dyes as neutrophils (72% of white cells), eosinophils (1.5%), and basophils (0.5%). The granulocytes are predominantly important in the removal of bacteria and parasites from the body. They engulf these foreign bodies and degrade them using their powerful enzymes.

Kinds of Granulocytes 

Neutrophils are phagocytic as well as granulocytic. Neutrophils use granulocytic chemicals to destroy microorganisms which they ingest. They have a multi-lobed nucleus. They are about 50-70% of circulating leucocytes (WBC )(higher numbers suggestive of bacterial infection). The fine granules stain poorly with acidic and basic dyes neutrophil. The primary granules are electron dense which contain bactericidal enzymes like Lysozyme, myeloperoxidase; neutral proteases (i.e. elastase); and acid hydrolases (B-glucoronidase). The secondary granules are smaller, not electron dense. They contain lysozyme, collagenase and lactoferrin and cathepsin B. Neutrophils are the 1st cells to arrive at the inflammatory site. They phagocytose and kill the ingested microorganisms in the phagosome.

Eosinophils release granulocytic chemicals into surrounding tissues, to destroy nearby infectious agents. They represent 1-3% of circulating WBCs. They possess a bi-lobed nucleus and a heavily granulated cytoplasm. Granules stain orange/red with the acidic dye Eosin Y. They are somewhat phagocytic but DO NOT act as APCs. The major role of the eosinophil is believed to be against parasites, particularly parasitic worms

.Basophils -  They have Lobed nucleus with large coarse granules stain blue with basic dye methylene blue. Like eosinophils, basophils release granulocytic chemicals, to destroy nearby microorganisms and stimulate the inflammatory response. They play a major role in the allergic response when they release their granules (containing histamine, serotonin, heparin, prostaglandin, etc into the bloodstream following exposure to specific allergens). Basophils bear Fc receptors for IgE . When an individual is exposed to an allergen, allergen specific IgE is produced. This IgE binds to the surface of basophils.

Lymphocytes

Lymphocytes are the main actors of the immune system and responsible for the specific immune responses. They represent 20-40% of circulating WBC in blood extravasate and enter the tissues – return 99% of cells in lymph. They are small 6µm, contain a single nucleus, little visible cytoplasm around their nucleus. Lymphocytes include T lymphocytes and B lymphocytes and natural killer cells.
T and B lymphocytes are small, motile, non-phagocytic cells which cannot be distinguished from each other morphologically. Once stimulated with antigen enlarges 15µm into a blast cell. Lymphoblasts further differentiate into effector cells or memory cells(Plasma cells, T-helper cells, T-cytotoxic cells). The memory cells are long-lived cells that reside in the Go phase of the cell cycle until activated by a secondary encounter with antigen. Different maturational stages of lymphocytes can be distinguished by their expression of membrane CD molecules (Cluster of Differentiation (CD).

T cells are chiefly responsible for cell mediated immunity. B cells are primarily responsible for humoral mediated immunity (relating to antibodies). T cells are named such because these lymphocytes mature in the thymus; B cells, named for the Bursa of fabricius in which they mature in bird species, are thought to mature in the bone marrow in humans. In the presence of an antigen, B cells can become much more metabolically active and differentiate into plasma cells, which secrete large quantities of antibodies.

Kinds of T lymphocytes

Helper T cells are responsible for activating and orchestrating the response against an invading organism. Cytotoxic (or Killer) T Cells are responsible for destruction of virus-infected or malignant (cancerous) body cells. Suppressor T cells are responsible for turning off the immune response after an infection has been cleared. T- helper cells interact with B- lymphocytes and help them to divide, differentiate and antibodies. T- helper cells interact with mononuclear phagocytes and help them destroy pathogens.
T- cytotoxic cells are responsible for the destruction of host cells infected with viruses or other intracellular pathogens.

B lymphocytes

B cells perform antibody mediated immunity. B lymphocytes have receptors on its surface to recognize one unique antigen. Antigens stimulate B cells to convert into plasma cells that produce antibodies (specific to that antigen). A plasma cell produces antibody molecules that can combine with a specific kind of antigen (like a lock & key). All antibodies eventually enter the blood or lymph. B-cells specialize in fighting bacterial invaders.
Plasma cells - B-cells on stimulation, grow, proliferate and differentiate into plasma cells. Plasma cells have a nucleus with chromatin arranged in a wheel-spoke pattern and an abundant, basophilic cytoplasm. Their extensive granular endoplasmic reticulum produces immunoglobulins. After completing secretion, most plasma cells die; however, some may survive for months or even years and serve as memory cells.
Natural killer cells (lymphocyte-like cells) attack & destroy virus-infected cells and cancer cells on first exposure to them. They lyse (rupture) cell membranes upon first exposure to these cells. NK cells provide an immediate, nonspecific defense. They act to slow the spread of viruses & cancer cells until the more specific adaptive immune cells can be mobilized.

Macrophages

These cells are derived from the bone marrow and have a variety of functions in the immune response. Macrophages are phagocytic cells responsible for engulfing and digesting foreign invaders. Examples of tissue-specific macrophages include: alveolar macrophages (in the lungs), mesangial Macrophages (in the kidneys), microglial cells (in the brain) and Kupffer cells (in the liver). In the tissues monocytes mature into different types of macrophages at different anatomical locations. Macrophages are activated by a variety of stimuli in the course of an immune response. One of the earliest activating signals comes from chemokines. Phagocyotosis itself is an important activating stimulus. Macrophages are further activated by cytokines secreted by T helper cells [IFN-gamma], by mediators of the inflammatory response and by various microbial products (such as LPS).
Macrophages play the following important roles: As scavengers, they rid the body of worn-out cells and other debris. They display bits of foreign antigen in a way that draws the attention of matching lymphocytes. They churn out an amazing variety of powerful chemical signals, known as monokines, which are vital to the immune responses.
1) phagocytosis
2) antimicrobial activity
3) secretion of soluble factors

Dendritic Cells

They originate in the bone marrow and function as antigen presenting cells (APC). They are found in the structural compartment of the lymphoid organs, in the bloodstream and other tissues of the body. They capture antigen or bring it to the lymphoid organs where an immune response is initiated. They stationed in skin and particularly susceptible to infection by viruses.

Mast cells (or mastocyte)

It is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Although best known for their role in allergy and anaphylaxis, mast cells play an important protective role as well, being intimately involved in wound healing and defense against pathogens. It is found in the lungs, skin, tongue, and linings of the nose and intestinal tract, where it is responsible for the symptoms of allergy. Mast cells are released from the bone marrow as undifferentiated precursor cells and do not differentiate until they enter the tissues (skin, connective tissue, mucosal epithelium, etc.). Mast cells bear Fc receptors for IgE (FcRs) and contain large numbers of cytoplasmic granules which also play a very important role in the allergic response. They produce a variety of cytokines. TNF is produced and stored within the cytoplasm of the mast cell, and it can be released quickly following mast cell activation.

The organs of the Immune System

Immunity is the ability of an animal to fight against infections and diseases. The immune system consists of a complex network of organs, cells, and molecules. They work together to defend the body from disease causing organisms (bacteria, viruses, fungi, and parasites). The immune system protects the body from pathogens and toxins that could disrupt the body  homeostasis.

Functions of immune system

1.       Provides defenses against pathogens

2.       Removes dead or worn out cells like RBCs

3.       Identifies and destroys abnormal cancer cells

4.       Protects from autoimmune diseases

5.       Rejects tissues cells of foreign antigens.

Characteristics of the immune response

1. Specificity: the ability to distinguish between antigens
2. Adaptiveness: the ability to respond to previously unseen molecules.
3. Recognition of self and non-self: the ability to recognize and respond to molecules that are foreign or ‘non-self ‘and the molecules that are ‘self’.
4. Memory: the ability to recall previous contact with a foreign molecule and respond to it in a learned manner.

The main features of Immune system includes pattern recognition (anomaly detection); distributed control; diversity, learning, memory, redundancy, robustness, feature extraction, multilayered and adaptive.

Parts of vertebrate Immune system

• Natural or innate or non-adaptive or non-specific immune system

• Acquired or adaptive or specific immune system

Immune Mechanisms

First lines of defense are the physical barriers which include the skin, urine, tears, cilia, mucosal membrane, etc. The skin provides an almost impenetrable biological barrier. Lysozyme is an enzyme found in tears and saliva that can break down foreign agents. Mucus and cilia found in the nose and throat can catch foreign agents then sweep them outside via coughing, sneezing and vomiting.
Second lines of defense are the macrophage system, complement, fever, interferon and inflammation. A second line of defense is the specific or adaptive immune system which may take days to respond to a primary invasion. The production of antibodies and cell-mediated responses may occur in which specific cells recognize foreign pathogens and destroy them. The response is often more rapid because of the activation of memory B and T cells. The cells of the immune system interact with one another by a variety of signal molecules. These signals may be proteins such as lymphokines, cytokines and chemokines which stimulate cells of the immune system
Third lines of defense are the specific system also known as acquired or adaptive immunity. The specific system consists of B cells (humoral), and T cells (cell-mediated).

Kinds of lymphoid Organs

Primary/central lymphoid organs  

The bone marrow and the thymus constitute the primary lymphoid organs. Both B-lymphocytes and T-lymphocytes are produced from stem cells in the bone marrow. B-lymphocytes mature in the bone marrow while T-lymphocytes migrate to the thymus and mature there. After maturation, both B-lymphocytes and T-lymphocytes circulate through and accumulate in secondary lymphoid organs.

Bone marrow

It is a central organ where all the immune cells are born and only B cells mature (process termed as B lymphopoiesis). THYMUS is the other central organ in which T cells mature (T lymphopoiesis).
Bone marrow is a spongy tissue found within bones. All blood cells are produced within the bone marrow. The bone marrow of an adult produces about three million red blood cells and 120,000 white blood cells every second. All these cells originate from a small population of stem cells, which may be as few as one in a million of the bone marrow cells.

Thymus

It is a flat, bilobed organ situated above the heart and below the thyroid gland. Each lobe is surrounded by a capsule and is divided into lobules. The lobules are separated from each other by strands of connective tissue called trabeculae. Each lobule is organized into two compartments: the cortex (outer compartment) the medulla (inner compartment). In the cortex, the network is densely packed with thymocytes. These cells are less dense in medulla. In medulla, the epithelial cells are more visible and Hassal's corpuscles are present (function unknown). The thymus is at its largest relative size at birth and its largest actual size is at puberty. After puberty the thymus begins to shrink. Precursor T cells enter thymus from the blood (there are no afferent lymphatic vessels) and mature into functional T lymphocytes.
The complete process of thymic education is a two-step process in which Thymic cortical epithelial cells function as the effector cells in a process known as positive selection. Positively selected thymocytes must go through a second phase of selection known as negative selection. The stromal cells secrete thymic hormones which are important in T cell maturation e.g., alpha- 1- thymosin, beta - 4- thymosin, thymopoietin and thymulin.

Secondary /peripheral lymphoid tissue

The secondary lymphoid organs include lymph nodes and the spleen, as well as lymphoid tissues. The lymphoid tissues include the tonsils, the appendix, and the Peyer's patches in the lining of the small intestines (gut-associated lymphoid tissue or GALT). The lymphoid tissue found beneath the mucous membranes of the bronchi (bronchial-associated lymphoid tissue or BALT). The lymphatic tissue found in the mucous membranes (mucosa-associated lymphoid tissue or MALT). The lymphatic tissue found beneath the skin (skin-associated lymphoid tissue or SALT).

Lymph nodes 

They are small, bean-shaped structures laced along the lymphatic vessels, with clusters in the neck, armpits, abdomen, and groin. Each lymph node contains specialized compartments where immune cells congregate, and where they can encounter antigens. The immune cells and foreign particles enter the lymph nodes via incoming lymphatic vessels. All lymphocytes exit lymph nodes through outgoing lymphatic vessels.
The lymph node consists of 3 regions: cortex is the outermost layer which contains mostly B lymphocytes, plus both follicular dendritic cells and macrophages all arranged in clusters called primary follicles. Following antigenic stimulation the primary follicles become secondary follicles consisting of concentric rings of densely packed lymphocytes, macrophages and dendritic cells. The germinal centers contain large proliferating B lymphocytes and plasma cells interspersed with macrophages and dendritic cells. The paracortex is the layer just beneath the cortex. This region is sometimes called the T dependent region of the lymph node. It is an area populated with T lymphocytes and also interdigitating dendritic cells. It is an important site for T cell activation by these APCs. The medulla is the inner most region, more sparsely populated by cells. Many of the cells are plasma cells; activated T helper and T cytotoxic cells are also present. In addition, there is a high concentration of immunoglobulin in this region due to the large population of plasma cells. The "swelling of lymph nodes" is the huge increase in the number of lymphocytes is due to: proliferation of lymphocytes within the lymph node, influx of lymphocytes from the circulatory system and antigenic stimulation.

Spleen

The spleen is the largest of the lymphoid organs which is located at the upper left of the abdomen. Like lymph nodes, the spleen contains specialized compartments where immune cells gather and work, and serves as a meeting ground where immune defenses confront antigens. It is surrounded by a capsule, which sends trabeculae into the interior to form a compartmentalized structure. There are two types of compartments: red pulp and white pulp. Red pulp is a network of sinusoids populated with macrophages and numerous erythrocytes. This is the site where old RBCs are destroyed and removed. White pulp surrounds the splenic arteries, forming a periarteriolar lymphoid sheath (PALS) populated mainly by T lymphocytes. Clusters of B lymphocytes in the white pulp form primary follicles occupying a more peripheral position. Upon antigenic challenge, these primary follicles develop into characteristic secondary follicles containing germinal centers.
The immune functions of spleen include: proliferation of lymphocytes, production of humoral antibodies and removal of macromolecular antigens from the blood.
The hematopoietic functions include: formation of blood cells in fetal life, removal and destruction of senile, damaged and abnormal RBCs and platelets and the retrieval of the iron from hemoglobin.

Lymphoid tissues 

They are clumps of lymphoid tissue are found in many parts of the body, especially in the linings of the digestive tract and the airways and lungs-territories that serve as gateways to the body. These tissues include the tonsils, adenoids, and appendix.Peyer's patches are areas of lymphoid tissue located in the wall of the intestine, and in some mammalian species such as sheep, cattle and rabbits. Peyer's patches have a function similar to the bursa of fabricius of birds and bone marrow of other mammals: B cell differentiation and maturation.

Functions of lymphatic organs

They generate immuno-competent lymphocytes, concentrate antigens into lymphoid organs, and circulate lymphocytes through lymphoid organs, exposing antigen to a variety of lymphocytes and deliver antibodies and effector T cells to the blood and tissues.

Visual Tools for effective Teaching and Learning

Visual learning is one of the best methods for teaching of thinking skills and helping to organize ideas. Visual diagrams help students make connections and transfer knowledge. They help students recognize patterns, interrelationships and inter-dependencies among and between new concepts. Visual diagrams create a “learn to think” and “learn to learn” attitude. “Learning to think” (how to think?) and “learning to learn”(how to learn?) are the essential skills of learner’s success. 

Proverbs, sayings and quotes on visual literacy

“One seeing is better than a hundred times telling about" - Old Chinese proverb.

“A picture is worth a thousand words” (an old adage).

“Every picture tells a story.” (an old saying).

“The use of visual tools creates a shift in classroom dynamics from passive to interactive learning for all to see.” (D. Hyerle, 1996).

Definition of visual literacy

Visual Literacy is “the ability to understand and use images including the ability to think, learn and express oneself in terms of images”.

Why visual literacy?

About 90% of all information that comes into our brain is VISUAL and about 40% of all nerve fibres connected to the brain are linked to the RETINA. About 36,000 visual images per hour may be registered by the EYES(From Brain Based Learning - Eric Jensen, 1996).

Visual displays can serve as a more effective alternative to traditional text presentation. Visual displays are effective for enhancing the learning of text information. Visual displays can serve to facilitate retention of information. Visual displays facilitate encoding and retrieval of information.

Visual intelligence

People learn visually by observing, perceiving, watching, connecting, drawing, showing, communicating and structuring their thoughts. The better you learn  to appreciate colour, harmony, balance, symmetry, repetition, direction and so forth, the more you’ll learn from what you see and the more you’ll enjoy life. Visualization is a two-way street. You can create pictures as well as look at them. Verbal literacy depends on learning grammar, rhetoric, and vocabulary. Visual literacy encompasses understanding color, tone, line texture, proportions, boldness, symmetry, repetition, accent and media.

Visual tools

Physical models, Mental pictures, interactive visualization, imaginary visualization and stories, pictures, sketches, diagrams, images and pictographs and Kinesthetic activity

Visual maps

 Brainstorming webs; graphic organizers; concept maps and mind maps

Benefits of visual tools

visual tools provide one mechanism to direct and focus attention. Visual tools give an instant picture of relationships of ideas and complex meaning. Visual tools improve presentation. Visual tools proved to be more persuasive. Visual tools facilitate recall and retention.

Visual learning helps students:

Visual learning helps brainstorm, develop, organize and communicate ideas. One can see connections, patterns and relationships in visuals. Students can assess and share prior knowledge. Visual learning can develop vocabulary. Visual diagrams can be useful for outlining ideas in the writing process. Students can highlight important ideas. They can clarify or categorize concepts, ideas and information. They can even comprehend the events in a story or book. Preparation of visuals improve social interaction between students and facilitate group work and collaboration among peers. They can facilitate recall and retention.Improve reading comprehensive skills and strategies.

Educational benefits of visual tools

            Graphic Organizers are visual representations of knowledge. It is a frame used to organize information. GOs are also referred to as: knowledge maps, story maps, cognitive organizers, advance organizers, concept diagrams and thinking maps. Visual tools help in organizing the thought process. They help rearrange text and useful in summarizing the information. They clarify thinking and locate and remember facts/ideas. They present the big picture or view information in a meaningful whole. They show inter-relationships among ideas.They reinforce understanding and integrate new knowledge.

Kinds of graphic organizers (GOs)

•          Bubble map

•          Double bubble map

•          Circle map

•          Chain of events

•          Clustering map

•          Cycle map 

•          Brace map

•         Venn diagram  

•          Interaction outline

•          Problem/solution

•          Persuasion map

•          Time – order chart

•          E-chart

•          T-chart

•          Flow chart

•          Tree chart           

Benefits of Graphic organizers as valuable instructional tools

              They help to organize, link, integrate and communicate large amounts of information more easily. They allow easy editing and drawing different perspectives. They are flexible and endless in application. They are underutilized in teaching - learning situation. They are ideal for many types of learners because GOs use short words or phrases. They offer greater retention and reduce instructional time. They can be used in all phases of learning from brainstorming ideas to presenting ideas.

Educational benefits of ICT tools and techniques

The world of Technology is advancing very rapidly and  is continuously changing.  Civilization can be identified by the stages of technology development i.e., stone age, bronze age, iron age, steam age, atomic age and computer age. Human civilization is technology dependent. Technology can be used as a tool for inquiry, a tool for construction, a tool for communication, a tool for expression, a tool for productivity and a tool to assist in problem solving and the making of informed decisions (Thomas et. al. 2002).

Definitions

Information technology (IT) is defined as the study or use of electronic equipment, especially computers for storing, analysing and sending out information.
Communication technology is the process of sending, receiving and exchanging information.
Information and communication technology (ICT) is a generic term referring to technologies, which are being used for collecting, storing, editing and passing on information in various forms (SER,1997). ICT literacy basically involves using digital technology, communication tools and/or access, manage, integrate, evaluate and create information in order to function in a knowledge society. 

The scope of Information and communication technology 

ICT is a major factor in shaping the new global economy and producing rapid changes in the society. The emergence of the ‘knowledge- based society’ is changing the global economy and status of education. In a short time, ICT has become one of the basic requirements of learning environment. ICTs provide students with skills to function effectively in this dynamic, information – rich and continuously changing environment. ICTs provide an array of powerful tools that may help in transforming the present isolated teacher- centered and text-book bound classrooms into rich, student-focused and interactive knowledge environment. ICT promotes the skill of learning to learn and improves their learning outcomes. ICT provides “head- heart and hand approach” to learning.

There are 3 aspects in technology based learning: learning from the technology, learning about the technology and learning with the technology.
Tools and techniques can be broadly defined as the practical methods and skills applied to specific activities to enable improvements. Tools are wonderful and powerful things. We use hundreds of them every day for a multitude of simple and complex purposes. \

Definition of technology and tools

Technology may be defined as the “systematic application of scientific or other organized knowledge to practical tasks” (Galbraith 1967). Technology is also referred to as the “application of scientific knowledge for human purpose.”

Technology communication tools- to collaborate publish and interact with peers, experts and audiences.
Technology research tools- to locate information, to process data and report results.
Technology productivity tools- construct models
Technology problem solving and decision- making tools

Kinds of  ICT tools

• ·Multimedia PC, laptop, notebook.
• CDs& DVDs. digital video, still camera.
• Internet and its tools- e-mail ,browsers, website, search engines, chat etc.
• Computer aided instruction& computer mediated conferencing, video/audio conferencing.
• Digital libraries , e-books& electronic publications.
• Interactive TVs.
• Microsoft publishing -news letter, poster, brochure.

ICT and education

ICT has transformed education. ICT caters to different learning styles. ICT helps students to gain valuable computer skills. ICT aids in collaboration and group work. ICT aids in the visualization of difficult concepts. ICT promote creativity. ICT enable multiplier effect of documents and communication. ICT provide flexibility and variety in learning. ICT provide a multimedia presence in the classroom.

Kinds of digital techniques 

• Word processing -documents, notes, projects, assignments
• Spread sheet programming -records, exam scores
• Data bases -information storage
• Graphing software -to prepare teaching-learning resources
• Developing Multimedia kits -to make process interesting
• Using internet and e-mail facilities -to gain knowledge
• Games and simulations -to improve quality of learning

Uses of information technology tools in learning 

Learners use computer-based services to search for and find relevant information in a range of contexts; Learners retrieve information using a variety of media; Learners decode information in variety of forms- written, statistical and graphic; Learners critically evaluate information,( the interconnectedness )of different fields of knowledge; Learners can use computer based services to write, analyse, present and communicate information; Learners can use information technologies to create networks of co-learners and to share, collaborate and construct knowledge.

Benefits of Computer based technology

Computer technology has undergone unparallel development. Computer capacity has doubled every 18 months, since the advent of the microprocessor 25 years ago.

1. Multi-sensory delivery: Text, graphics, animation, sound and video.
2. Active learning: enhanced learner interest.
3. Co-operative learning: positive group/social interaction.
4. Communication Skills: one-to-one, one-to-many, many-to-many.
5. Multicultural education: link students from across the country/around the world.
6. Motivation: greater learner engagement (time).

College Autonomy

Autonomy in colleges is an opportunity to the teachers and students to make innovations, utilize their creative talent, improve the standards of teaching, examination and research and quickly respond to social needs.An autonomous college will take up the responsibility of the academic programmes, the content and quality of teaching and the admission and the assessment of students.

The concept of autonomy was meant to promote academic independence as well as excellence.  It also encouraged the introduction of innovations in order to improve standards of education,quality assurance and higher academic standards. Being an examination oriented system, teaching is to a certain extent subordinated to examinations.  Testing and evaluation must help towards assessing several dimensions of the learner.. Autonomy when exercised with the sense of responsibility and accountability will inevitably lead to excellence in academics, governance and financial management of the colleges.

Institutional autonomy

The institutional autonomy is the functional status given to the colleges by the UGC which includes three important aspects: academic autonomy, administrative autonomy and financial autonomy. Academic autonomy is the freedom to decide academic issues like curricula, instructional materials, pedagogy and student evaluation techniques. Administrative autonomy is the freedom to manage its own affairs with regard to administration. Financial autonomy is the freedom to the institutions to utilize the financial resources at its disposal in a prudent way keeping in view of its priorities.

The responsibility of  Autonomous College

An autonomous college will be fully accountable for the content and quality of education that it imparts.  The students would receive greater individual attention on the basis of their needs and aptitude.  Autonomy would encourage the students to think clearly, critically and creatively and to express themselves effectively. 

Origin of autonomy

The Kothari Commission (1964 – 1966) considered autonomy a must for intellectual development and had recommended the following criteria:

• Freedom in curriculum design.
• Adoption of new teaching – learning methods
• Revision of rules for admission.
• Implementation of separate evaluation methods.
• Introduction of specific programmes. 

UGC Guidelines for Autonomous Colleges

They will have freedom to determine and prescribe its own courses of study and syllabi. They can prescribe rules for admission in consonance with the reservation policy of the state government. They can evolve methods of assessment of student work, the conduct of examinations and notification of results. They can use modern tools of educational technology to achieve higher standards and greater creativity.

Accountability of Autonomy

Accountability can be defined as the obligation of an individual or organization to account for its activities, accept responsibility for them and to disclose the results in a transparent manner. Accountability is for personal, financial and physical resources in relation to the specific academic objectives and overall national development.

External accountability may include

o   Analysis of contents of the courses.

o   Course options.

o   Co-curricular and extra-curricular activities.

§  Performance of students.

§  Students’ employment.

o   Contribution to generation of knowledge and

o   Teachers’ contribution to extension etc. 

Internal accountability may include

1.       Resource acquisition.

2.         Efficiency index.

3.        Average work load.

4.         Average time distribution between lectures, tutorials /practicals.

5.         Group discussions.

6.         Project work.

7.        Teaching aids used.

8.       Programmes and activities planned and implemented.

9.       Professional development of teachers.

10.    Utilization of infrastructural facilities.

11.    Number of books / journals in the library.

The concept of autonomy was meant to promote academic independence as well as excellence. It also encourages the introduction of innovations in order to improve  standards of education.

‘Our higher education has to be internationally comparable in quality’   
– Rastogi Report