Antibody diversity is generated by multiple mechanisms, each of which contributes to the generation of antibodies with up to 10^11 different amino acid sequences in their antigen-binding sites. Several of these mechanisms involve changes in the DNA sequences encoding the antibody heavy and light chain proteins. One mechanism that does not rely on changes to the DNA within the immunoglobulin heavy and light chain gene loci is, instead, dependent on:

Correct Answer: A

Rationale: Heavy/light chain pairing is post-DNA; VDJ , κ/λ , and TdT alter DNA.

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.

Correct Answer: B

Rationale: Somatic hypermutation introduces mutations in V genes to boost affinity, occurring in germinal centers.