Diagnostic urpose Since the immune system recognizes and remembers virtually all ag that are introduced into an individual, assays which demonstrate the presence of ab to an organism in the serum of a patient have become a standard way of determining that the patient has had contact with, was infected by, the organismSince the immune system recognizes and remembers virtually all Ag that are introduced into an individual, assays which demonstrate the presence of Ab to an organism in the serum of a patient have become a standard way of determining that the patient has had contact with, was infected by, the organism Diagnostic purpose