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Abstract
The immune system is a remarkably versatile defence system that has evolved to protect animals from invading pathogenic micro-organisms and cancer. It is able to produce a variety of cells and molecules capable of specifically recognizing and eliminating an apparently unlimited variety of foreign invaders thus providing immunity.
The immune response has two components which interact together in a highly organized manner to eliminate a foreign invader. An innate one providing the first line of defence against infection. Most of its components are present before the onset of infection and are not specific to a particular pathogen but rather to classes of molecules peculiar to frequently encountered pathogens. Phagocytic cells as macrophages and neutrophils, barriers such as the skin and a variety of antimicrobial compounds synthesized by the host play important roles in innate immunity.(1)
In contrast, the more specific component, adaptive immunity, does not come into action until there is an antigenic challenge to the organism. It responds with a high degree of specificity and exhibits the remarkable property of memory, so that subsequent responses to a particular pathogen occur quicker, are stronger and more effective in clearing it. It is also capable of self/nonself recognition and responds only to nonself antigens. This is essential to avoid the fatal consequences of autoimmunity. The key players here are the lymphocytes and their products including the antibodies.(1)
Two major populations of lymphocytes have been recognized; namely B and T cells. Together they constitute about 20-40% of the body’s leukocytes with an absolute count of about 1011 lymphocytes equally divided between the B and T cell compartments.(1) B lymphocytes recognize extracellular (including cell surface) antigens, and differentiate into antibody secreting cells (plasma cells), thus are responsible for mediating the humoral immunity. T lymphocytes, the cells responsible for cell-mediated immunity, recognize the antigens of intracellular microbes and function to destroy them or the infected cells. They do so through their antigen receptors (T-cell receptor (TCR)). T lymphocytes have a restricted specificity for antigens; they recognize only peptide antigens attached to host proteins that are encoded by genes in the major histocompatibility complex (MHC) expressed on the surfaces of other cells; the antigen presenting cells (APCs).(2)