Practice question

Blood grouping depends on detecting antigens naturally present on erythrocyte membrane such as A, B and RhD. When specific antiserum is mixed with whole blood, IgM antibodies bind directly to intrinsic surface antigens without requiring artificial carrier coupling. This direct agglutination occurs because red cells themselves act as particulate antigen, forming visible clumps without intermediate coating step. Passive agglutination involves antigens attached to latex or red cells secondarily, useful for soluble antigens. Fluorescent or double variants are not standard for routine ABO typing. Hence direct method remains reference for transfusion compatibility.