Biology · Body Fluids and Circulation · NEET
Antigens (A and B) sit on the surface of the RBCs. Antibodies (anti-A and anti-B) float in the plasma. NCERT states ABO grouping is based on two surface antigens on the RBCs, while the plasma of different people contains two natural antibodies. Remember: antigen = on the cell, antibody = in the fluid.
The rule is that a person never carries an antibody against an antigen they already have, or their own blood would clump. AB has both A and B antigens, so it can carry neither anti-A nor anti-B (no antibody). O has no antigen, so it safely carries both anti-A and anti-B antibodies.
An antigen is a chemical (a surface molecule) that can trigger an immune response; here A and B antigens are on the RBC surface. An antibody is a protein made in response to an antigen; anti-A and anti-B are present naturally in plasma. Antigens are on cells, antibodies are proteins in plasma.
If donor RBCs carry an antigen that matches the recipient's plasma antibody, the antibody binds and clumps those RBCs. For example, A-group blood (A antigen) given to a B-group person (who has anti-A) causes agglutination. This is why the antigen-antibody pair must not match between donor and recipient.
Group O confuses students most: it is called the universal donor because it has NO antigen, yet its plasma has BOTH antibodies. Students wrongly think 'O = nothing.' O has no antigen but both anti-A and anti-B antibodies.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Two antigens (A and B) on RBCs and two antibodies (anti-A and anti-B) in plasma.
Group A has A antigen on its RBCs and anti-B antibody in its plasma.
Group AB has no antibody because it carries both A and B antigens, so it cannot carry anti-A or anti-B.
Group O has no antigen on its RBCs, but its plasma contains both anti-A and anti-B antibodies.
They are natural antibodies present in plasma from birth, unlike Rh antibodies, which form only after exposure to Rh-positive blood.
