Most normal tissues contain resident macrophages, and connective tissue sites in the gastrointestinal tract and the lung contain large numbers of these cells. Yet the blood also contains a high number of circulating 'classical' monocytes that can differentiate into macrophages after entering tissues. These circulating monocytes function to:

Correct Answer: D

Rationale: Monocytes enter infected tissues to become macrophages ; blood phagocytosis , lining , or lymph node patrol are less primary.

Correct Answer: D

Rationale: A heart attack (myocardial infarction) involves cardiac muscle damage due to a blocked coronary artery, not brain tissue death (which is a stroke).

Correct Answer: B

Rationale: Antibodies from breast milk provide passive immunity, transferred without the infant's immune system activation, unlike active or nonspecific immunity.

Correct Answer: C

Rationale: Condoms reduce HIV transmission, preventing AIDS; no vaccine or simple cure existed in 2012, and survival varies beyond 2-3 years with treatment.

Correct Answer: D

Rationale: NK cell cytotoxicity involves activating signals, lack of inhibition (e.g., low MHC), and death receptor ligands (e.g., FasL), making all options correct.