C3a and C5a can cause

Correct Answer: B

Rationale: The correct answer is B: vascular permeability. C3a and C5a are anaphylatoxins that can cause increased vascular permeability, leading to fluid leakage and inflammatory responses. They do not directly cause bacterial lysis (choice A), phagocytosis of IgE-coated bacteria (choice C), or aggregation of C4 and C2 (choice D). Increased vascular permeability is a key mechanism by which these complement proteins contribute to inflammation and immune responses.

Correct Answer: A

Rationale: Activated helper T cells release various cytokines to regulate the immune response. Interleukin-1 is primarily produced by macrophages and monocytes, not helper T cells. It plays a role in inflammation and fever. Interleukin-2, interleukin-4, and gamma interferon are indeed released by activated helper T cells to stimulate immune responses. Therefore, choice A, interleukin-1, is the correct answer as it is not typically released by activated helper T cells.

Correct Answer: B

Rationale: The correct answer is B because the antibody-binding site is primarily formed by the hypervariable regions of both the heavy (H) and light (L) chains. These regions contain the most sequence diversity and directly interact with antigens. Choice A is incorrect because the constant regions do not directly participate in antigen binding. Choice C is incorrect as it only mentions H chains, disregarding the contribution of L chains. Choice D is incorrect as it focuses solely on the variable regions of H chains, neglecting the importance of the L chains in forming the binding site.

Correct Answer: D

Rationale: The correct answer is D. The affinity for antigen is determined by the variable region of the antibody, specifically the antigen-binding site. The heavy-chain constant region influences the other properties listed. A: The ability to cross the placenta is determined by the structure of the heavy-chain constant region, particularly the Fc region. B: Isotype (class) is determined by the type of heavy-chain constant region present in the antibody. C: The ability to fix complement is also influenced by the heavy-chain constant region, particularly the Fc region. Therefore, D is the only property that is not dependent on the structure of the heavy-chain constant region.

Correct Answer: D

Rationale: The correct answer is D because macrophages are responsible for processing antigens and presenting them to T cells, initiating an immune response. They are antigen-presenting cells that help activate T cells. Choice A is incorrect because macrophages do not make antibodies. Choice B is incorrect as macrophages do not lyse virus-infected cells; this is typically done by cytotoxic T cells. Choice C is incorrect as macrophages do not directly activate cytotoxic T cells but rather present antigens for T cell activation.