NBME Immunology Practice Test

Assessing the overall functional activity of the classical complement pathway is best done with a test that measures the end result of the entire cascade. The CH50 assay does exactly that: it quantifies the serum’s ability to lyse antibody-sensitized sheep red blood cells, which requires intact classical pathway components from C1 through C4 and C2 to form the C3 convertase and drive hemolysis. By finding the serum dilution that yields 50% lysis, the CH50 reflects the functional integrity of the classical pathway; a low CH50 indicates a deficiency or dysfunction in any classical component. In contrast, the AH50 tests the alternative pathway, and measuring a single protein like C3 by ELISA or C5 by Western blot assesses quantity rather than the pathway’s overall functional activity.