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.