Whenever an antigen enters through parenteral route it can initiate antibody production, combine with corresponding specific antibody and produce mainly five types of reactions, such as precipitation, agglutination, neutralization, opsonization, and complement fixation. Precipitation is an antigen-antibody reaction in which the antigen is in soluble form and when corresponding antibody is added it gets precipitated and the complex of antigen and antibody settles down at the bottom of the test tube leaving a clean supernatant fluid, with its corresponding antibody only when they are combined in optimum proportions, which varies from antigen to antigen, and precipitation will not occur when there are excess antibodies or excess antigen. Agglutination is a type of antigen-antibody reaction in which the antigen is in particulate form and when it is mixed with corresponding antibody clumping of the particulate antigen takes place in the presence of electrolyte like NaCl. That is why agglutination tests are done by suspending the particulate antigen in normal saline. Neutralization test is a type of antigen-antibodies reaction in which the toxic effects or the pathogenic effects of an antigen are neutralized by specific antibodies.