This article will be presented in two parts, concluding next week.
INTRODUCTION
Antigens and superantigens are proteins produced by viruses, bacterium, or other microbes. While the body's immune system can dispose of antigens without harming healthy tissue, superantigens stimulate an abnormally strong reaction by the body's immune system. They can literally stimulate the body's immune system into frenzied activity. Superantigens stimulate a marked expansion of T-cells that have receptors on the skin.
ANTIBODIES
Antibodies are one of two important antigen recognition molecules of the immune system. The other antigen recognition molecule is found on the T cell, called the T cell receptor (TcR). Antibodies also act as receptors on B cells but usually we think of antibodies as the soluble factor secreted by plasma cells. Soluble, or secreted, antibody is structurally slightly different than the antibody on the surface of B cells but the antigen recognition sites are similar.
There are five different classes of antibodies or immunoglobulins (Ig) known as IgD, IgA, IgM, IgE and IgG. There are four subclasses of IgG and two sub-classes of IgA. Each of the antibody classes will be considered separately but first consider the basic function of antibodies. Then we will examine the structure and how the genes for these proteins make the amazing diversity of these proteins possible.
Antibodies recognize antigen. The ability of antibodies to recognize specific antigen is an important characteristic. Antigen recognition and binding allows antibodies to perform three important functions.
Opsonization is a process where bacteria, virus-infected cells and others are tagged for destruction. Antibodies are the tags (or opsonins) because one end (Fab) binds to an antigen on the surface of the organism. The other end of the antibody (Fc) binds to receptors on phagocytic cells like macrophages and neutrophils. The antibody signals the phagocyte to engulf and destroy the organism or cell. Antibodies may also stimulate extra-cellular killing with an oxidative burst and the release from phagocytes of free radicals and destructive enzymes. Complement components, like C3b, also act as opsonins (C3b opsonization).
Antibodies also help to kill foreign organisms by means of an activating complement. IgM and IgG are very good at this, resulting in the lysing of the target by the C5-9 complex (MAC attack).
In the circulation and in tissues, antibody will bind to toxins to remove them by forming easily recognized antibody-antigen complexes, which are phagocytosed in the liver and elsewhere. Antibodies can also immobilize and agglutinate infectious agents by binding to surface of antigens and prevent them from attaching to sites of infection like the intestinal mucosa or from penetrating cells as in the case of a virus.
