Introduction to Immunology-Part II - Page 2

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Page 2

Jan 2, 2001

Helper T cells are also known as CD4 cells in the literature. They hold a central role in most aspects of immunity. After binding to the Class 2 MHC- antigen complex, the helper T cells secrete interleukin, which is a type of cytokine. There are two types of helper T cells; Th1 cells activate macrophages to destroy antigens they have ingested and stimulate the proliferation of cytotoxic T cells, while Th2 cells stimulate B cells to proliferate and secrete antibodies. The helper T cells secrete many other cytokines that perform multiple roles in immunity.

Cytotoxic T cells are also known as CD8 cells or killer T cells. As part of the normal renewal process of cells, proteins in the cytosol compartment are digested and broken down into short fragments called peptides.These peptides are attached to Class 1 MHC molecules and brought to the cell surface. If the peptides are derived from an infecting virus or an abnormal protein synthesized by the cell, the cytotoxic T cell will recognize the peptides as antigens and react to them. Since the only effective way to deal with an infected or abnormal cell is to destroy it, the cytotoxic T cell secretes molecules called porins that break down the cell.

B Lymphocytes

B lymphocytes complete their development within the bone marrow. The antigen combining sites on B cells are called antibodies. There are five isotypes or classes of antibodies. Since they are composed of a type of protein called immunoglobulin, they are designated IgM, IgG, IgE, IgA and IgD. Antibodies are composed of four chains arranged in a Y-type structure. The antigen-binding site is located in the so-called variable region, and the idiotype refers to the structural differences of this site that allows it to bind to a specific antigen. After maturation, IgM is bound to the surface of the B cell. The primary immune response involves the initial encounter with antigen. The B cell secretes IgM, which combines with the antigen. Several antigens clump together forming a particle that is very susceptible to phagocytosis. At the same time, the complement system is activated. Since this process is not the most effective immune response, a secondary response is initiated after receiving a stimulus of interleukin from the helper T cells. As a result, the B cells multiply, change their antibody type to that which will most effectively eliminate the antigen, and differentiate into either plasma or memory cells. IgG has the highest antibody concentration in the blood, and has the broadest application. IgG together with complement is very efficient in promoting phagocytosis by coating the antigen. IgE and IgA have specialized functions in the immune response. The final stage of the secondary response is the differentiation of B cells into plasma cells or into memory cells. B cells specialize I the secretion of vast amounts of antibodies.

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