The membrane IgM and IgD on the surface of an individual B cell

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: 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.

Correct Answer: B

Rationale: The correct answer is B: it provides antibody more rapidly. Passive immunization involves giving pre-formed antibodies directly to an individual, providing immediate protection. This is advantageous over active immunization, where the body needs time to produce antibodies after exposure to an antigen. Choice A is incorrect because passive immunization is typically administered through injection, not orally. Choice C is incorrect as the duration of antibody persistence is usually shorter in passive immunization compared to active immunization. Choice D is incorrect because passive immunization can contain various types of antibodies, not just IgM.

Correct Answer: D

Rationale: Rationale for Correct Answer (D): Haptens are small molecules that can bind to antibodies without inducing an immune response on their own. They require a carrier protein to become immunogenic and induce an antibody response. However, haptens do not need to be processed by CD8+ cells to become immunogenic, as they primarily induce responses from B cells. CD8+ cells are involved in processing antigens for presentation to T cells. Therefore, statement D is incorrect. Summary of Incorrect Choices: A: True - haptens can bind to antibodies. B: True - haptens require a carrier protein to induce antibody response. C: True - both penicillin and poison ivy allergens are haptens.