Grafts between genetically identical individuals (i.e., identical twins)

Correct Answer: C

Rationale: The correct answer is C because genetically identical individuals have the same major histocompatibility complex (MHC) antigens, so there is no immune response or rejection of grafts. Minor histocompatibility antigens (choice A) are not relevant in this case. Hyperacute rejection (choice B) occurs due to pre-existing antibodies, which is not a factor in genetically identical twins. Choice D is incorrect as both kidney and skin grafts would be accepted between identical twins due to genetic similarity.

Correct Answer: B

Rationale: The correct answer is B because membrane-bound IgM and IgD on a B cell have identical variable regions (VH and VL) but different constant regions (CH). This is important for B cell receptor diversity while maintaining specificity. Choice A is incorrect as they have different heavy and light chains. Choice C is incorrect because they have different constant regions. Choice D is incorrect as they have identical variable regions.

Correct Answer: D

Rationale: The correct answer is D because when adult AB spleen cells are injected into a newborn B mouse, the recipient's immune system is still developing. This allows the injected cells to establish tolerance towards strain A antigens. This process is known as neonatal tolerance induction. Option A is incorrect because the immune system of the recipient is not mature enough to mount a strong response to destroy the injected cells. Option B is unlikely as the injected cells would likely interact with the recipient's immune system. Option C is less likely as the immune system of the newborn mouse is not fully developed to initiate a graft-versus-host reaction efficiently.

Correct Answer: B

Rationale: The correct answer is B because cytotoxic T cells recognize and kill target cells that are infected by the specific virus that induced their activation. This is achieved through the recognition of viral antigens presented on class I MHC molecules. Therefore, the target cells must be infected by virus A and have antigens presented on class I MHC molecules that are identical to the T cells' receptors. Choice A is incorrect because cytotoxic T cells are specific to the virus that induced their activation, not any virus from the same host. Choice C is incorrect because cytotoxic T cells primarily recognize antigens presented on class I MHC, not class II MHC. Choice D is incorrect because the target cells should be infected with the same virus that activated the cytotoxic T cells, not a different virus.

Correct Answer: C

Rationale: The correct answer is C because the purified protein derivative (PPD) is used to test for tuberculosis, which elicits a delayed-type hypersensitivity reaction. Helper T cells and macrophages are key players in this response, as they release cytokines and activate macrophages to combat the antigen. Eosinophils (A) are more commonly seen in allergic reactions, neutrophils (B) are prominent in acute bacterial infections, and B cells (D) are involved in antibody production, not the primary response to PPD.