Which one of the following substances is NOT released by activated helper T cells?

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: 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.

Correct Answer: D

Rationale: The correct answer is D because allotypes are indeed due to genetic polymorphism within a species. Allotypes are genetic variants of immunoglobulins, resulting from differences in amino acid sequences. This genetic diversity within a species allows for variability in the immune response. Choices A, B, and C are incorrect. Allotypes can be found on both heavy and light chains, they are not determined by class I MHC genes, and they are not confined to the variable regions.