Prozone phenomenon is a feature is -

In an agglutination test, a person's serum (which contains antibodies) is added to a test tube, which contains a paicular antigen. If the antibodies agglutinate with the antigen to form immune complexes, then the test is interpreted as positive. However, if too many antibodies are present that can bind to the antigen, then the antigenic sites are coated by antibodies, and few or no antibodies directed toward the pathogen are able to bind more than one antigenic paicle. Since the antibodies do not bridge between antigens, no agglutination occurs. Because no agglutination occurs, the test is interpreted as negative. In this case, the result is a false negative. The range of relatively high antibody concentrations within which no reaction occurs is called the prozone. Examples include high levels of syphilis antibodies in HIV patients or high levels of cryptococcal antigen leading to false negative tests in undiluted samples. This phenomenon is also seen in serological tests for Brucellosis. The serological test is mainly seen in the precipitation reaction. Reff: Ananthanarayan &Parker's textbook of microbiology 9th edition pig no.: 105,342