What are antigens, antibodies, humoral and cell mediated immune response


Antigens:
A recognition system allows the mammalian immune system to distinguish ‘self’ from ‘non self’. Prior to birth, the 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 the foreign material leading to its removal. Foreign (“non self”) substances 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 (F×b) for specific invaders (i.e. antigens). The tail of Y can activate the complement system (20 distinct defensive proteins in serum) or bind to receptors on phagocytic cells.

Antibody-Mediated (Humoral):
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 human system (e.g. macrophages).

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

The action of NK cells in why tissue and organ transplants are rejected in birds or mammals.
Rejection mechanism: In rejection mechanism NK cells enter the transplanted tissue through the blood vessels, recognize it as foreign, attach to the tissue and destroy it. The body tolerates tissue and organ transplants between identical twins that have identical sets of DNA and hence identical self recognition marker regions.

T cells also regulate the activity of other parts of the immune system. For example, suppose several bacteria penetrate the first line of defense (the skin) through an abrasion or cut. Inflammation occurs and macrophages phagocytize the bacteria. Macrophages destroy most of antigens (bacteria) but some of the antigens are moved to the surface of the plasma membrane of both macrophages and B cells, where they are displayed alongside the self recognition marker. It is this specific combination of self recognition marker and antigen that helper T cells recognize.

When the macrophage reacts with a helper T cell, it releases interleukin – 1 (IL – 1). Interleukin I stimulates other helper T cells to secrete interleukin – 2 (IL – 2), which stimulates their (helper T cells) growth and cell division. Some of the interleukin – 2 that helper T cells produce acts as sensitized B cells. Sensitized B cells have recognized and processed the antigen onto their plasma membrane alongside their self recognition makes. These stimulated B cells mature and divide into differentiated plasma cells.

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