This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
Antigen antibody precipitation is maximally seen in which of the following? 
Select and explain. 
A. Excess of antibody, B. Excess of antigen, C. Equivalence of antibody and antigen, D. Antigen Hapten interaction
C: Equivalence of antibody and antigen
Answer: C: Equivalence of antibody and antigen 
Explanation: Ans. is 'c' i.e., Equivalence of antibody and antigen Precipitation reactionIt is a type of antigen antibody reactionWhen a soluble antigen combines with its antibody in the presence of electrolytes (NaCl) at a suitable temperature and pH, the antigen antibody complex forms an insoluble precipitate.The amount of precipitate formed is greatly influenced by the relative proportions of antigens and antibodies.Precipitation occurs most rapidly and abundantly in the zone of equivalence in which the antigen and antibody are present in optimal proportions.If increasing quantities of antigens are added to the same amount of antiserum in different tubes and the amounts of precipitate in different tubes are plotted on a graph, the resulting curve will have three phases: