PNAd is expressed on

Correct Answer: A

Rationale: The correct answer is A: HEV (High Endothelial Venules). PNAd is a ligand expressed on high endothelial venules, which helps in the recruitment of lymphocytes from the bloodstream into secondary lymphoid organs. PNAd is not primarily expressed on lymphocytes themselves (B) or dendritic cells (C). While PNAd can be involved in thymocyte migration to the thymus, it is not expressed on the thymus itself (D). Thus, the correct answer is A as it directly relates to the function and location of PNAd expression.

Correct Answer: C

Rationale: The correct answer is C: gp120. This is because gp120 is a key HIV antigen that plays a crucial role in binding to CD4 receptors on T cells, facilitating viral entry. By targeting gp120, vaccines can induce antibodies that block this interaction, preventing HIV from infecting CD4+ T cells. Conversely, choices A, B, and D (gp41, Reverse transcriptase, Matrix proteins) are not as effective targets for interfering with the CD4+ T cell interaction, as they do not directly impact the initial binding and entry process of HIV into the cells.

Correct Answer: C

Rationale: The correct answer is C: TNF-alpha and TNF-beta. Tumor necrosis factors (TNF) are a specific category of cytokines known for their role in promoting inflammation and inducing cell death in tumor cells. TNF-alpha and TNF-beta are two key members of this cytokine category. Interleukins (choice A) are a different group of cytokines involved in immune responses. Interferons (choice B) are cytokines that help regulate the immune system's response to viral infections. TGF-beta (choice D) is a cytokine involved in regulating cell growth and differentiation, but not specifically associated with tumor necrosis factors.

Correct Answer: C

Rationale: The correct answer is C because Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns (PAMPs) on pathogens, initiating the immune response. This recognition triggers the production of cytokines and chemokines, leading to inflammation and activation of immune cells. Choice A is incorrect because TLRs do not identify self-antigens but rather foreign antigens. Choice B is incorrect as TLRs do not directly mediate phagocytosis but rather initiate signaling pathways that lead to phagocytosis by immune cells. Choice D is incorrect because TLRs do not suppress cytokine secretion; rather, they stimulate cytokine production.

Correct Answer: B

Rationale: The primary function of lymph nodes is to filter lymph and facilitate immune responses. Lymph nodes contain immune cells that help recognize and fight off infections. They filter lymph by trapping and removing pathogens and foreign particles. This process helps in activating the immune response to protect the body. Choices A, C, and D are incorrect because lymph nodes do not produce red blood cells, synthesize antibodies, or generate cytokines. These functions are carried out by other organs and cells in the body.