The B cell co-receptor, composed of CD19/CD21/CD81, is a receptor that binds to complement fragments such as C3dg. When an antigen bound by the BCR on a B cell has also been tagged with C3dg, the B cell co-receptor is stimulated together with the BCR. Signaling through the co-receptor:

Correct Answer: C

Rationale: Co-receptor boosts BCR via PI 3-kinase ; not inhibition (A, B) or Src alone .

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.

Correct Answer: D

Rationale: Central tolerance (thymus) removes self-reactive T cells, supplemented by peripheral tolerance, preventing autoimmunity.