Prozone

In an agglutination or precipitation reaction, prozone or prezone is the zone of relatively high antibody concentrations within which no reaction occurs.

In an agglutination test, a person's serum (which contains antibodies) is added to a test tube which contains a particular antigen. There are many types of different antibodies present in a persons serum, and there might be one particular antibody that will bind to antigen present in the tube. There are also other types of antibodies which would not bind the antigen. The antibody-antigen complex forms an agglutinate.

In some cases, the concentration of antibody specific for a particular antigen is too low compared to other types of antibodies, and when this occurs, the small portion of agglutination that took place will be masked by larger aggregate of non-binding antibodies, and the mixture remain as fluid. The agglutination which took place is invisible, and this is called prozone. This can lead to false negative result. Therefore in each agglutination test, dilution of the antibody-antigen mixture is done to a certain level, until agglutination can be seen, or otherwise the test is negative.