Antigens are soluble molecules and are larger in size in precipitation reactions. Precipitation reactions differ from agglutination reactions in the size and solubility of the antigen. Excess of either component reduces lattice formation and subsequent precipitation. These reactions depend on the formation of lattices (cross-links) when antigen and antibody exist in optimal proportions. They are based on two soluble reactants that come together to make one insoluble product, the precipitate. Precipitation reactions are based on the interaction of antibodies and antigens. When instead of sedimenting, the precipitate remains suspended as floccules, the reaction is known as flocculation. Thus, for precipitation reactions to be detectable, they must be run in the zone of equivalence. In the post zone phenomenon, small aggregates are surrounded by excess antigen and again no lattice network is formed. This is because antigen combines with only a few antibodies and no cross-linkage is formed. In the prozone phenomenon, there are too many antibodies for efficient lattice formation. The zone of antibody excess is known as the prozone phenomenon and the zone of antigen excess is known as the post zone phenomenon. On either side of the equivalence zone, precipitation is actually prevented because of an excess of either antigen or antibody.
Equivalent Antigen and Antibody: Maximum amount of lattice (Precipitate) is formedĪntigen and antibody reaction occurs optimally only when the proportion of the antigen and antibody in the reaction mixture is equivalent.Antibody access: Too much antibody prevents efficient crosslinking/lattice formation.Antigen access: Too much antigen prevents efficient crosslinking/lattice formation.Antigen and antibody must be in an appropriate concentration relative to each other. The antigen must be bivalent or polyvalent that is it must have at least two copies of the same epitope or different epitopes that react with different antibodies present in polyclonal sera. The antibody must be bivalent a precipitate will not form with monovalent Fab fragments. The formation of an antigen-antibody lattice depends on the valency of both the antibody and antigen. The interaction of antibodies with soluble antigens may cause the formation of an insoluble lattice that will precipitate out of the solution. Antibodies that aggregate soluble antigens are called precipitins. A lattice is formed between the antigens and antibodies in certain cases, it is visible as an insoluble precipitate.
This reaction is called a precipitation reaction. When a soluble antigen reacts with its specific antibody, at an optimum temperature and P H in the presence of electrolyte antigen-antibody complex forms insoluble precipitate. Precipitation Reaction is a type of antigen-antibody reaction, in which the antigen occurs in a soluble form.