The classic complement pathway is initiated by interaction of C1 with

Correct Answer: C

Rationale: The correct answer is C: antigen-IgG complexes. The classic complement pathway is initiated when C1 binds to antigen-IgG complexes. This interaction triggers a cascade of enzymatic reactions leading to opsonization, inflammation, and cell lysis. The other choices are incorrect because A (antigen) does not directly activate the complement pathway, B (factor B) is part of the alternative pathway, and D (bacterial lipopolysaccharides) activate the lectin pathway, not the classic pathway.

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.

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.