Antigen antibody precipitation is maximally seen in which of the following? (AIIMS 10)
|A||Excess of antibody|
|B||Excess of antigen|
|C||Equivalence of antibody and antigen|
a. Equivalence of antigen and antibody- when a soluble antigen combines with its antibody in the presence of electrolytes at a suitable temperature and pH, the complex forms an insoluble precipitate.
b. The amount of precipitate formed is greatly influenced by the relative proportions of antigens and antibodies.
c. Precipitation results when a large lattice is formed consisting of alternating antigen and antibody molecules. This is possible only when antigen and antibody are present in optimal/ equivalent proportions.
d. Excess of antibody-A/C prozone
e. Excess of antigen-A/C postzone: In either of these two cases, precipitation is weak or absent, as the lattice does not enlarge. This is called the zone phenomenon. The importance of the zone phenomenon lies in sera with antibody excess which may give false negative serological reaction, unless several dilutions are used.
f. Antigen-hapten interaction- haptens are different from antigen, as antigens stimulate antibody production too. Haptens do not interact with antigen but their presence can inhibit precipitation of specific antibodies by their corresponding antigen.
Serial or doubling dilutions in Widal test, Standard Agglutination test for brucella or Weil Felix test is done to overcome prozone phenomenon.