The principal difference between cytotoxic (type II) and immune complex (type III) hypersensitivity is

Correct Answer: B

Rationale: The correct answer is B because the principal difference between cytotoxic (type II) and immune complex (type III) hypersensitivity lies in the site where antigen-antibody complexes are formed. In type II hypersensitivity, the complexes form on the surface of target cells, leading to cell destruction. In contrast, in type III hypersensitivity, the complexes form in circulation or in tissues, leading to immune complex deposition and subsequent tissue damage. The other choices are incorrect because the class of antibody (A), participation of complement (C), and participation of T cells (D) can be common features in both type II and type III hypersensitivity reactions, but the crucial distinguishing factor is where the complexes are formed.

Correct Answer: D

Rationale: The correct answer is D: decreased resistance to Neisseria bacteremia. This is because C6 is a component of the complement system, which plays a crucial role in the immune response against Neisseria bacteria. A deficiency in C6 would impair the ability to effectively combat Neisseria infections. Choice A is incorrect because the deficiency of C6 does not directly affect resistance to viral infections. Choice B is incorrect as hypersensitivity reactions are not specifically linked to C6 deficiency. Choice C is incorrect because there is no direct correlation between C6 deficiency and an increased frequency of cancer.

Correct Answer: B

Rationale: The correct answer is B because helper T cells play a crucial role in activating B cells to produce antibodies. In this scenario, the protein component of the hapten-protein conjugate must be recognized by helper T cells to initiate the immune response. This recognition leads to the activation and differentiation of B cells into plasma cells that secrete anti-hapten antibodies. Choices A, C, and D are incorrect because the focus should be on the protein component, not the hapten or suppressor T cells, for the production of anti-hapten antibodies.

Correct Answer: D

Rationale: The correct answer, D, is not true because class I MHC antigens are primarily found on all nucleated cells, not just B cells, macrophages, and activated T cells. These antigens present endogenous antigens to CD8+ T cells. Choice A is incorrect because class I MHC antigens are not typically assayed using antibody and complement; instead, they are recognized by CD8+ T cells. Choice B is incorrect because identifying class I MHC antigens in the lab usually requires more than a few hours due to the complexity of the process. Choice C is incorrect because class I MHC antigens are controlled by a single gene locus in the major histocompatibility complex.

Correct Answer: C

Rationale: The correct answer is C because classic-pathway C3 convertase does not contain C3b. In the classical pathway, C3 convertase is formed by the cleavage of C4bC2a, which does not involve C3b. A: classic-pathway C5 convertase contains C3b as it is formed by the binding of C4b2a3b complex to C5. B: alternative-pathway C5 convertase contains C3b as it is formed by the binding of C3bBb3b complex to C5. D: alternative-pathway C3 convertase contains C3b as it is formed by the binding of C3bBb complex to C3.