hemagglutination

Hemagglutination is the clumping of red blood cells (RBCs) in suspension caused by cross-linking of cells through multivalent interactions with agglutinins, such as antibodies, or with viral surface proteins that bind to RBC receptors. In immune-mediated hemagglutination, antibodies directed against RBC surface antigens (often IgM or IgG) bind to antigens on multiple cells, forming a lattice that causes visible clumping. In viral hemagglutination, certain viruses express surface hemagglutinin proteins that bind to receptors on the erythrocyte membrane, leading to agglutination. This property is characteristic of several viruses, including influenza, some parainfluenza viruses, and others.

Laboratory use of hemagglutination centers on serological and virological assays. The hemagglutination assay assesses the ability of a test sample to cause RBCs to agglutinate, with results reported as a titer based on the highest serum dilution that yields agglutination. The hemagglutination inhibition assay measures antibodies that can block viral hemagglutinin from binding RBCs, thereby preventing agglutination; this test is widely used to quantify functional antibodies against viruses such as influenza and to evaluate vaccine responses.

Clinical and diagnostic relevance includes blood typing, where anti-A and anti-B reagents cause agglutination of donor RBCs that bear the corresponding antigens. Hemagglutination can also occur in nonimmune contexts, such as cold agglutinin disease, where IgM autoantibodies cause RBC clumping at low temperatures; warming typically reverses the agglutination. Overall, hemagglutination is a foundational concept with applications in transfusion medicine, immunology, and infectious disease diagnostics.