Antigens and Antibody, Humoral and cell mediated immune response and Primary and Secondary Immune Responses


Antigens and Antibody:
A recognition system allows the mammalian immune system to distinguish ‘self’ from ‘non self’. Prior to birth body inventories the proteins and various other large molecules present in the body (“self”) and inactivates most of the genetic programming for making antibodies to self molecules.

The body can distinguish self molecules from foreign non self substances and lymphocytes can produce specific immunological reactions against foreign material leading to its removal. Foreign (“non self”) substances (markers) to which lymphocytes respond are called antigens (antibody generator). Most antigens are large proteins or other complex molecules with a molecular weight generally greater than 10,000.

Antibodies: Plasma cells manufacture antibodies (immunoglobulins) a group of recognition glycoproteins present in the blood and tissue fluids of birds and mammals. All antibody molecules have a basic Y structure composed of four chains of polypeptides connected by disulphide bonds. The arms of Y contain binding sites or fragments (Fab) for specific invaders (e.g. antigens). The tail of Y can activate the complement system.

Humoral and cell mediated immune response:
Humoral immune system (Antibody mediated immune response): B cells are of great importance in fighting invading organisms because they produce antibodies that identify the antigens for destruction. B cells carry some of their particular antibodies on their plasma membrane. When an antigen comes into contact with B cell whose antibodies recognize the antigen, B cell binds to the antigen when stimulated by one kind of T cell (a helper T cell), B cell divides many times producing plasma cells that produce and secrete more of this particular antibody. These antibody molecules are carried through the circulation. If they encounter antigens, the antibodies bind to the antigen molecules and thus mark them for destruction of other parts of immune system (e.g. macrophages).

Cell mediated immune response: T cells are directly involved in destroying invading cells as well as in regulating other parts of immune system. T cell responses are called cell mediated immune response. Natural Killer (NK) cells also called cytotoxi T cells, recognize cell surface changes on cancer cells, virus infected cells, fungi, bacteria, protozoa or Helminth parasites.

Primary and Secondary Immune Responses:
Primary immune response: Primary immune response is the response an animal generates during its first encounter with an antigen. During this response, the antigen disappears from the blood because it is either bound to antibodies or phagocytized by macrophages. Most of B cells producing antibodies also die.

Secondary immune response: If the same antigen enters the body second time, body mounts secondary immune response that is faster and more extensive than the primary response. This rapid response is possible because the immune system has stored a ‘memory’ of antigen. Some of the original B cells differentiated into memory cells that remain in the body for life. Memory cells from B cells clones continually expose antibodies on their plasma membrane surface. If the antigen invades the body again these memory cells rapidly produced large number of antibody secreting B cells.

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