General processes of the immune response

The formation of antibodies is only one mechanism by which an animal may protect itself from substances or microorganisms that are potentially harmful. A mechanical protection against infection is provided by the presence of an intact skin surface and membranes together with the secretion of mucus from many internal membrane surfaces. The acids secreted by the stomach and skin have a bactericidal effect as does the presence in many body fluids of certain enzymes, particularly lysozyme. 

Invasion of the tissues by an infective agent initiates an inflammatory response in the animal. This is non-specific and is mediated primarily by substances released from tissues that are damaged as a result of either trauma or the toxic effects of the infective agent. The major mediator is the vasoactive amine histamine, which causes an increased local blood flow and capillary permeability, resulting in local oedema. A major aspect of the inflammatory response is the involvement of large numbers of phagocytic cells, particularly the polymorphonuclear leucocytes. These are chemotactically attracted to the inflamed tissues and are mainly responsible for the elimination of particulate material. This often results in the destruction of many of these cells and the formation of pus. 

One very important group of cells known as the lymphocytes, which are widely distributed throughout the tissues, appear in increased numbers during an inflammatory response and are primarily responsible for the immune response. This is a specific response to the invading substance or agent by the animal and involves the production of cells and antibodies with the ability to recognize and bind the invading substance. 

Immunologically competent cells, whether they are T or B lymphocytes, have membrane receptors that are specific for an antigen. It is basically the binding of the antigen to the specific receptor on the appropriate lymphocyte which initiates the whole process, stimulating the cell to proliferate and producing a clone of identical cells, a process known as clonal selection . The nature of the secondary response is due in the main to this large number of cells now available. 

Antibodies are members of a group of proteins known collectively as immunoglobulins. The name is derived from the observation that during electrophoresis of blood plasma the proteins associated with antibody activity migrate with the gamma globulin fraction. 

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