B-cell antigen receptors can interact directly with antigen, whereas T-cell antigen receptors only recognize antigen when it is presented to them on the surface of another cell.

Correct Answer: A

Rationale: B-cell receptors (BCRs) bind free antigens directly, while T-cell receptors (TCRs) require antigen presentation via MHC molecules on another cell's surface.

Correct Answer: D

Rationale: Neutrophils are primary phagocytes that engulf and kill bacteria using bactericidal mechanisms.

Correct Answer: D

Rationale: All three (macrophages, dendritic cells, B cells) are APCs critical for antigen uptake and presentation to T cells; since E is moved to D, D is correct for all.

Correct Answer: C

Rationale: MHC genes are highly polymorphic, providing diverse antigen presentation across populations, unlike BCR or TCR genes.

Correct Answer: D

Rationale: Penicillin is a small hapten that requires conjugation to a carrier protein to elicit an immune response.