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Agglutination occurs when antibodies in the recipient's blood recognize antigens on donor red blood cells as foreign. The antibodies bind to these antigens, causing the red blood cells to clump together. This antigen-antibody reaction is the immune system's rejection response to incompatible blood, making agglutination a visible indicator of blood type incompatibility during transfusion.

The agglutination test mixes a blood sample with specific antibodies. Type A blood clumps with antibody-A, type B with antibody-B, and type AB with both antibodies. Type O blood shows no clumping because it lacks surface antigens. Similarly, the Rh blood group is detected using D-antibodies; positive clump formation indicates Rh-positive blood, while no clumping indicates Rh-negative.

Type O blood lacks both A and B antigens on red blood cell surfaces, so it does not trigger immune reactions in recipients with any blood type. Because type O blood cannot be recognized as foreign by antibodies in type A, B, or AB recipients, it can be safely transfused to all blood types without causing agglutination or immune cross-reactivity.

Type AB recipients are universal recipients because they lack antibodies against both antigen A and antigen B. They can receive blood from all blood types without immune reaction. However, as donors, type AB blood can only be given to other AB recipients, since their blood contains both A and B antigens that would trigger immune responses in recipients with type A, B, or O blood.

Rh-positive individuals can receive both Rh-positive and Rh-negative blood safely. However, Rh-negative individuals can only receive Rh-negative blood to avoid immune cross-reactivity. If an Rh-negative person receives Rh-positive blood, their immune system produces antibodies against the D-antigen, potentially causing hemolytic disease in future pregnancies or transfusions.

When incompatible blood is transfused, the recipient's immune system detects foreign antigens and produces antibodies against them. This antigen-antibody reaction causes agglutination, where red blood cells clump together. The clumped cells can block blood vessels, leading to serious complications including hemolysis, organ damage, and potentially fatal immune reactions if the incompatibility is severe.

Type O individuals produce antibodies against both antigen A and antigen B. Although their blood lacks antigens and can be given to anyone, they cannot receive type A, B, or AB blood because their antibodies would attack the incoming antigens, causing agglutination and immune rejection. They can only safely receive type O blood from other universal donors.