Recombination signal sequences are conserved heptamer and nonamer sequences that flank the V, J, and D gene segments which undergo recombination to generate the final V region coding exon. Some of these have 12-nucleotide spacers between the heptamer and nonamer, and others have 23-nucleotide spacers. The reason recombination signal sequences come in these two forms is:

Correct Answer: A

Rationale: 12/23 rule ensures V-D-J assembly ; not helix face , locus specificity , or non-Ig genes .

Correct Answer: C

Rationale: Vaccines often use weakened or inactivated pathogens to stimulate immunity, not antibodies, B cells, or antibiotics.

Correct Answer: B

Rationale: HIV targets helper T cells, not mast cells (involved in allergies), making B false; others are true.

Correct Answer: A

Rationale: NK cells, of lymphoid origin, target altered host cells (e.g., virus-infected or cancerous), not bacteria or antigens directly.

Correct Answer: B

Rationale: CD40 ligation by T cells activates B cells to start the germinal center reaction, key for affinity maturation.