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Antigen antibody reactions
The interaction of an antigen determinant and antibody molecule is called immune complex or antigen - antibody complex. Various factors influencing antigen-antibody complex. Specificity antibody to combine with only one type of antigen, Binding site of antigen and antibody ( epitope and paratope), Binding forces of antigen and antibody – closeness between antigen and antibody and intermolecular forces, Affinity (attraction of Ag- Ab binding ) and Avidity (combining capacity of heterogenous antibodies with multivalent antigen).
The first interaction of an antigenic determinant (epitope) with its corresponding antigen binding site on an antibody is called a primary antigen- antibody reaction. The primary antigen-antibody reactions are rapid reaction, not dependent on electrolytes and not visible. If the primary antigen- antibody reaction is followed by the aggregation of antigen antibody complexes into macroscopically visible clumps is called the secondary antigen-antibody reaction and this aggregation phase may take hours to day to reach maximum. The two visible reactions are called precipitation and agglutination.
Precipitation is the combination of soluble antigen with specific antibody, which leads to the formation of an insoluble aggregation. Immune precipitation occurs when antigen and antibody combine in solution and form a visible aggregate. Precipitation reaction is quantifiable. The variation in the ratio of antibody- antigen leads to different levels of lattice formation, and thereby to different amounts of precipitate. This phenomenon, called the prozone phenomenon were antibody may excess, zone of equivalence of antigen- antibody or antigen may excess. Factors affect precipitation are temperature, pH, salt concentration and reaction volume.
The clumping, or agglutination, of particulate antigens by specific antibodies. Clumping results in the formation of a lattice in which antigen and antibody are cross linked. Agglutination methods are qualitative or semi quantitative at best and its reaction can be used in many applications as it posses a high degree of sensitivity. Agglutination reactions can be classified as either direct or indirect. In the direct agglutination reaction, the antigenic determinant is a normal constituent of the particle surface. In the indirect agglutination a molecule is ordinarily soluble is attached to a particle and rendered insoluble.
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