Rh factor is a protein on the surface of red blood cells.

• A. A carbohydrate on the surface of red blood cells
• B. A lipid on the surface of red blood cells
• C. A protein on the surface of red blood cells
• D. A protein in plasma

Answer: (C)

Rh factor refers to the D antigen, a protein that sits in the membrane of red blood cells. It is a protein anchored on the cell surface, not a carbohydrate or lipid, and not something circulating freely in plasma. Because it's a surface protein, the presence or absence of this antigen on RBCs defines whether blood is Rh positive or Rh negative. This contrasts with ABO antigens, which are carbohydrate structures on the cell surface. Clinically, the D protein on RBCs can provoke an immune response if an Rh-negative person is exposed to Rh-positive blood, leading to the production of anti-D antibodies.