Friday, December 25, 2009

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.


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.


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.

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