Infections can be diagnosed by detection of an immunologic response specific to an infecting agent in a patient’s serum. Normal humans produce both IgM (first-response antibodies) and IgG (antibodies that may persist long after an infection) to most pathogens. For most pathogens, detection of IgM antibodies or a fourfold increase in the patient’s antibody titer is considered to be diagnostic of current infection. If the infecting agent is rare or previous exposure is unlikely (e.g., rabies virus, botulin), the presence of specific IgG antibody in a single serum specimen can be diagnostic. Methods for detecting the presence of antibodies include immunodiffusion assay, complement fixation, enzyme-linked immunosorbent assay (ELISA), indirect or direct fluorescent antibody, radioimmunoassay, and Western blot immunoassay (see Chapter 8).