Scaffold proteins are often phosphorylated at multiple sites, allowing the recruitment of several different signaling proteins. In antigen receptor signaling pathways, this mechanism is used to bring enzymes in close proximity to their substrates. Termination of this signaling mechanism would be most efficiently accomplished by:

Correct Answer: D

Rationale: Dephosphorylation disrupts scaffold signaling; ubiquitination is slower, GAP or depletion are less direct.

Correct Answer: C

Rationale: Memory cells respond to antigens in the secondary immune response, triggering a faster, stronger reaction, not hormones, antibodies, or plasma cells.

Correct Answer: B

Rationale: A second vaccination boosts memory cells, increasing antibody levels faster and stronger, not involving plasma cell survival or circulation.

Correct Answer: D

Rationale: Secondary responses are rapid (short lag) due to memory cells, stronger than primary, and don't generate new memory cells.

Correct Answer: A

Rationale: T cells mature in the thymus, unlike B cells (bone marrow); the thyroid is unrelated.