NBME Immunology Practice Test

This tests a sensitive serologic method used to detect specific antibodies or antigens in serum, typically employed as a screening test with a confirmatory step. In ELISA, the target antigen or antibody is captured on a solid surface, and an enzyme-labeled secondary antibody produces a color change that reflects the amount of target present. Its high sensitivity makes it ideal for screening large numbers of samples for infections or immune markers. Because ELISA can yield false positives, a more specific test is used to confirm positive results. Western blot fits this role well: it separates viral or bacterial proteins by electrophoresis, transfers them to a membrane, and uses patient antibodies to reveal which specific proteins are recognized. The pattern of bands provides higher specificity, helping distinguish true infections from cross-reactivity or nonspecific binding. Other methods like immunofluorescence can detect antibodies but are less practical for high-throughput screening, and agglutination tests are less sensitive. Western blot is typically reserved as a confirmatory step rather than a primary screening test.