ANTIGEN - ANTIBODY REACTION

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BASIC PRINCIPLE OF SERO-IMMUNOLOGY

ANTIGEN-ANTIBODY REACTION

Serology is primarily concerned with antigen-antibody reactions in vitro as they apply to the diagnosis of disease.  All the tests done in serology involve the principle of antigen-antibody reactions.

Antigen is a complex chemical substance found in disease producing agents.  It has been used in two senses, first when introduced into the body will cause the production of antibodies, and second, to denote a substance which reacts in an observable or visible way with antibodies.  There are a number of substances which although they react with antibodies have not been shown to produce antibodies when injected into animals.  A substance is not said to be antigenic unless it will cause the production of antibodies.

Conditions of antigenicity:

1. 
   
   Antigen must be foreign to the circulation of the animal's body.


2. 
   
   Antigen must have a minimal degree of complexity and a certain molecular size (generally 10,000 or higher).


3. 
   
   Entrance of antigen into the animal body by surface injury or natural openings.


4. 
   
   Specificity of the antigen.


5. 
   
   Rigid structure of the determinant group.

An antigen has two distinct parts: the greater part which is composed of the carrier portion and is protein in nature and the other one is the determinant group or haptene.  It is usually non-protein and made up of carbohydrates, lipids, amino acids and simple inorganic radicals.  As a whole, most antigens are pure proteins, although some have lipoprotein or carbohydrate protein complexes.  Example is the pneumococcus capsule, composed of a pure polysaccharide, which functions as an effective antigen.

An antibody is a substance, globulin in nature, which is produced by the body and capable of reacting with antigen in some visible way.  One way of detecting antibodies is by their power of neutralizing infectious agents (may be bacterial, viral, rickettsial) or rendering them susceptible to destruction.  Formation of antibodies takes place in the reticulo-endothelial system - bone marrow, spleen, liver, and lymph tissue.  Antibodies have been observed to cause swelling of capsules of microorganisms against which they are directed - to cause lysis, to cause agglutination, to cause the formation of precipitate when mixed with soluble antigen, to phagocytize microorganisms, to neutralize toxic products of bacteria (or other toxins) and to sensitize animals passively to various antigens.  The chemical and physical properties of antibodies are the same with that of normal serum globulin, particularly gamma globulin.  Antibodies have a molecular weight of 150,000 to 200,000.  They are usually found in blood or lymph and also the cerebrospinal fluid.  The ability to form antibodies is influence by the following factors -- genetic constitution, age, sex and hormonal status.  While exact mechanism of production of so-called normal antibodies is still unknown, they may be due to:

1. 
   
   Ingestion and absorption of undenatured antigens (in foods) related antigenically to those occuring in infectious agent.


2. 
   
   Absorption of related antigens in the air from respiratory tracts.


3. 
   
   Absorption of infectious agents themselves or closely related agents.


4. 
   
   Production of antibodies by active immuninization or sub-clinical infections in which case they are actually acquired.

Two conditions are usually found in antigen-antibody reactions: cross reactivity and zonal reactions.  Antigens have a high order of specificity, in which they stimulate the body tissues to produce antibodies which will react to them alone and with no other antigen.  However, it sometimes occurs that different disease-producing agents have the same antigen and this incites the production of similar antibody.  This leads to false positive reaction in serological test.  Another thing, a single organism contains a variety of antigenic substances and is therefore capable of producing different antibodies -- this leads also to false positive reaction.

There are other things which produce false negative results, that is, whenever there is an excess antibody and small amount antigen and this leads to a visible reaction called "Prozone reaction." Likewise, when there is an excess antigen and small quantity of antibody, a phenomenon called "Post zone reaction" occurs.

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