Direct hemagglutination occurs in which of the following scenarios?

• A. Antibody-coated latex particles agglutinate with soluble antigen.
• B. Agglutination inhibition occurs when soluble antigen is present.
• C. Autoantibodies are detected by coating cells with antigen.
• D. Antibodies in patient serum cause RBCs bearing corresponding antigens to agglutinate directly.

Answer: (D)

Direct hemagglutination happens when antibodies in the patient's serum bind to antigens on the surface of red blood cells and cross-link neighboring cells, causing the RBCs to clump together. The antigens are already present on the cell surface, so the antibody acts as a bridge between cells, forming a visible agglutination without any added carriers or soluble components. This is the typical mechanism seen in ABO typing, where red cells themselves provide the antigenic targets and the corresponding antibodies in serum induce clumping.

The other scenarios describe different assay formats: using antibody-coated latex particles to agglutinate with soluble antigen represents a coated-carrier (passive) agglutination approach; agglutination inhibition occurs when soluble antigen blocks the reaction, not direct cross-linking of cells; and detecting autoantibodies by coating cells with antigen is an indirect method that does not rely on antibodies cross-linking native RBCs.