Consider a patient with blood type A with antigen-A on their red blood cells. Naturally, this patient’s blood has no antibodies against antigen A.

If this patient is injected with blood of type B, their immune system will detect the antigen-B in the incoming blood as foreign, and produce antibodies against it, leading to the rejection of the blood transfusion.

Using this antigen-antibody reaction, the agglutination test can detect a patient’s blood type.

A few drops of type A blood, when mixed with serum-containing antibody A, will form visible clumps by a process called agglutination. Similarly, type B blood forms clumps with antibody-B.

Type AB blood contains both antigens, therefore, it agglutinates with both antibodies A and B.

Since type-O blood has no antigens on the surface of red blood cells, it does not react with antibody-A or antibody-B. Therefore, clumps do not form.

Similarly, the Rh blood group is detected using D-antibodies. Positive clump formation indicates Rh-positive blood, and a negative result infers Rh-negative.

During a blood transfusion, the donor and recipient’s blood must be compatible to avoid immune cross-reactivity.

Type A blood cannot be given to an acceptor with type B, or O, because the acceptors’ blood produces antibodies against incoming antigen-A, leading to an immune reaction.

Due to the lack of antigen, blood type-O does not elicit immune reactions with other blood types. Therefore type-O blood can be given to patients with all other blood types - hence type-O patients are known as universal blood donors. However, they can only receive blood from patients with type-O blood, otherwise, they will produce antibodies against type A or B.

Conversely, a person with blood type-AB has no antibodies to either antigen A or B. Therefore, they are known as universal blood recipients. As donors, however, their blood should only be given to other patients of AB type, to avoid eliciting an immune reaction.

Similarly, a person with Rh-positive blood can be given both Rh-positive and Rh-negative blood, but a person with Rh-negative can only be given Rh-negative blood to avoid immune cross-reactivity.