T cells that express γ δ chains:

Correct Answer: A

Rationale: γδ T cells have innate-like roles, often lack CD4/CD8, and recognize non-MHC antigens.

Correct Answer: D

Rationale: All (A, B, C) can cause autoantibodies; thus, D is correct.

Correct Answer: C

Rationale: IgG is abundant in newborns via maternal transfer; IgM rises later.

Correct Answer: A

Rationale: Innate immunity is present at birth; acquired (B, C) develops later.

Correct Answer: D

Rationale: Attenuated means weakened. While making live attenuated vaccine the virulence of the microorganism is lost but its antigenic characteristics are retained.