Erythrocytes transport oxygen and,

A. AB, Rh-negative: AB RBCs carry both A and B antigens, so they cannot serve as universal RBC donors (but AB plasma is often considered a universal plasma donor).
B. O, Rh-negative: O negative RBCs lack A, B, and Rh (D) antigens, so they are the universal donor for red blood cell transfusions.
C. AB, Rh-positive: AB positive RBCs have A, B, and Rh antigens and therefore are not universal RBC donors.
D. O, Rh-positive: O positive RBCs lack A and B antigens but express Rh (D) antigen, so they can’t be given safely to Rh-negative recipients; therefore they are not fully universal donors.

A. initiate blood clotting: Platelets and coagulation factors initiate clotting; erythrocytes do not initiate clot formation.
B. transport nutrients: Plasma carries most nutrients (glucose, amino acids, lipids), not erythrocytes.
C. defend the body against pathogens: White blood cells (leukocytes) are the main defenders; erythrocytes do not have immune functions.
D. transport some carbon dioxide: Erythrocytes carry a portion of CO₂ (dissolved as bicarbonate via carbonic anhydrase inside RBCs and also some carbaminohemoglobin).
E. regulate erythropoiesis: Erythropoiesis is regulated mainly by erythropoietin (from kidneys) and iron/nutritional signals, not by erythrocytes themselves.