Which of the following represents the correct sequence of host immune defenses as encountered by a pathogen?

Correct Answer: B

Rationale: The correct sequence is B: Physical barriers; innate responses; adaptive responses. Physical barriers like skin and mucous membranes are the first line of defense, followed by innate responses (e.g., inflammation, phagocytosis) that provide immediate, non-specific protection. Adaptive responses (e.g., antibody production, T cell activation) are the final defense mechanism, specifically targeting the pathogen. Choices A, C, and D are incorrect because they do not follow the correct chronological order of how host immune defenses are encountered by a pathogen. Option A and D have the innate and adaptive responses in the wrong order, while option C places adaptive responses before physical barriers, which is not accurate in terms of the sequence of host immune defenses.

Correct Answer: D

Rationale: The correct answer is D because M cells do not transport IgA from the lamina propria into the intestinal lumen. Instead, they transport antigens and pathogens from the gut lumen to underlying immune cells. Choice A is correct as M cells are found in Peyer's patches. Choice B is correct as M cells are specialized epithelial cells. Choice C is incorrect as M cells have microfolds on their luminal side, not microvilli.

Correct Answer: A

Rationale: The correct answer is A: CD4+ T cells. HIV primarily infects CD4+ T cells by binding to the CD4 receptor and a co-receptor, usually CCR5 or CXCR4. CD4+ T cells are crucial for coordinating the immune response. B cells (choice B) are not typically infected by HIV. CD8+ T cells (choice C) can be indirectly affected by HIV, but they are not the primary target of the virus. Therefore, the correct choice is A as it accurately reflects the primary target cells of HIV infection.

Correct Answer: B

Rationale: Correct Answer: B (VDJ gene rearrangement) Rationale: 1. VDJ gene rearrangement occurs during B cell development in the bone marrow, independent of T helper cells. 2. Somatic hypermutation and immunoglobulin class switching require T helper cell interactions. 3. Memory cell differentiation also involves T helper cells for activation and differentiation. 4. Therefore, VDJ gene rearrangement is the only process not directly dependent on antigen-specific T helper cells.

Correct Answer: A

Rationale: Neutrophils are typically not found in the lamina propria of the gut in the absence of infection because they are primarily involved in the acute inflammatory response to infections. Macrophages, plasma cells, and T cells are commonly present in the gut lamina propria to maintain immune homeostasis and respond to antigens. Neutrophils are more abundant in tissues during active infection due to their role in phagocytosis and inflammation. Therefore, the correct answer is A: Neutrophils.