In serologic diagnosis, which immunoglobulin test is most informative?

• A. IgM against surface proteins
• B. IgA against capsule antigens
• C. IgG against phase I and II antigens
• D. IgG against a single phase

Answer: (C)

In serologic diagnosis of brucellosis, the most informative test looks for IgG antibodies against both phase I and phase II antigens because these two phases reflect different parts of the organism’s antigenic makeup and different stages of infection. Antibodies against phase I antigens are associated with exposure to the virulent, smooth form, while phase II antigens elicit responses that can appear as the infection evolves. Measuring IgG to both phases captures a broader immune response, increasing sensitivity across acute and chronic phases and improving specificity by distinguishing a comprehensive antibody profile rather than a response to a single antigen. Testing IgG against only one phase may miss infections where the dominant antibody response targets the other phase, reducing diagnostic yield. The other options—IgG against a single phase, or IgM or IgA against other antigens—either reflect a narrower window of the immune response or can be less reliable due to timing or cross-reactivity.