The predominant antibody isotype in external secretions that can neutralise pathogens and toxins and prevent their interaction with mucosal surfaces is

Correct Answer: A

Rationale: IgA dominates in secretions (e.g., saliva, mucus), neutralizing pathogens at mucosal surfaces.

Correct Answer: C

Rationale: J-chain links IgM and IgA multimers ; not T-cells , neutrophils , or IgE .

Correct Answer: D

Rationale: Attenuated microbes are live, weakened for vaccines; toxoids are inactivated toxins.

Correct Answer: B

Rationale: T cells mature in the thymus ; produced in bone marrow.

Correct Answer: D

Rationale: Pattern recognition receptors (PARs) are genomically determined to bind to molecules widely expressed by microbes but not by host cells. Consequently, PARs cannot recognize host-associated molecules such as MHC class I molecules or cells of host origin such as B, T, or natural killer lymphocytes.