NK cells can be activated following recognition of a virus-infected cell, if that cell has down-regulated expression of MHC class I proteins on its surface. However, NK cells can also recognize infected cells or tumor cells, even if they still express MHC class I proteins. In this latter case, activating receptors on NK cells are recognizing:

Correct Answer: A

Rationale: Stress molecules (e.g., MICA) activate NK cells ; cytokines , decoys , or mutations are less common triggers.

Correct Answer: B

Rationale: Complement proteins enhance inflammation by attracting immune cells and promoting pathogen destruction, not inducing antibodies, NK cell activity, or replacing T cells.

Correct Answer: B

Rationale: Antibodies agglutinate pathogens, clumping them for easier removal, not destroying complement, solubilizing, or crystallizing.

Correct Answer: A

Rationale: Pepsin, a digestive enzyme in the stomach, acts as a gut-specific chemical barrier, unlike AMPs and lysozymes, which are broader innate defenses.

Correct Answer: D

Rationale: Antigen generation involves peptides for MHC, via exogenous, endogenous, or cross-presentation pathways, essential for adaptive immunity.