Introduction to Immunology-Part II
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
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