During blood transfusion, agglutination may occur as a result of the reaction between

Analyze the question: It asks what reaction causes agglutination (clumping of red blood cells) during a blood transfusion.

Concept: Red blood cells carry antigens (A and/or B) on their surface. Plasma carries antibodies (anti-A and/or anti-B). Agglutination happens when an antigen meets its matching antibody, for example antigen A with anti-A, or antigen B with anti-B.

Blood groups, for reference:

• Group A: antigen A, anti-B antibody
• Group B: antigen B, anti-A antibody
• Group AB: antigens A and B, no antibodies
• Group O: no antigens, anti-A and anti-B antibodies

A person's own blood never contains a matching pair, otherwise it would clump. So agglutination happens only when donor and recipient are mismatched. For example, if group A blood is given to a group B person, the donor's antigen A meets the recipient's anti-A, and the cells clump.

Evaluate the options:

• A. "Contrasting antigens and antibodies": this is the wording that is open to confusion. Antigen A and anti-B (contrasting, or non-matching) do not react. The reacting pairs are antigen A with anti-A, and antigen B with anti-B.
• B. Two different antigens: antigens do not react with each other. Wrong.
• C. Two different antibodies: antibodies do not react with each other. Wrong.
• D. Similar antigens and antibodies: this means the corresponding antigen and antibody (A with anti-A, B with anti-B). This is the one that causes agglutination. Correct.

Answer: D

Tip: Remember "matching pair = clump." Antigen A + anti-A, or antigen B + anti-B, causes agglutination. This is also why group O (no antigens) is the universal donor and group AB (no antibodies) is the universal recipient.