Which of the following statements is true about the ABO blood group system?

Step 1: Analyze the question We need the true statement about where antigens and antibodies are found in the ABO blood group system.

Step 2: Key concept

• Antigens (A and B) are protein/carbohydrate markers found on the surface of red blood cells.
• Antibodies (anti-A and anti-B) are found dissolved in the blood plasma.
• A person never carries antibodies against their own antigens. If they did, their blood would clump (agglutinate).

| Blood group | Antigens on red cells | Antibodies in plasma | |---|---|---| | A | A | anti-B | | B | B | anti-A | | AB | A and B | none | | O | none | anti-A and anti-B |

Step 3: Check each option

• A. Antigens on white blood cells. False. In the ABO system, the antigens are on red blood cells.
• B. Antibodies in the blood plasma. True. Anti-A and anti-B antibodies circulate in the plasma.
• C. Antibodies on the surface of red blood cells. False. The red cell surface carries antigens, not antibodies.
• D. Antigens in the blood plasma. False. The antigens are on red cells, not in the plasma.

Step 4: Answer The correct option is B.

Exam tip: Remember "Antigens are on the Red cells, Antibodies are in the plasma." Think of Antigens on the Armour (the cell surface), and Antibodies Afloat in the liquid plasma.