Secondary lymphoid tissues are where:

Correct Answer: C

Rationale: The correct answer is C because secondary lymphoid tissues, like lymph nodes and spleen, are where lymphocytes encounter antigens and become activated. This process leads to the proliferation and differentiation of lymphocytes, enabling them to mount an immune response against specific pathogens. Choices A, B, and D are incorrect because A pertains to primary lymphoid tissues, B describes a localized immune response, and D refers to the process of gene rearrangement that occurs in the primary lymphoid tissues during lymphocyte development.

Correct Answer: C

Rationale: The correct answer is C because celiac disease primarily involves an abnormal adaptive immune response to gluten, not an innate response. Here's the rationale: 1) Celiac disease is characterized by a T-cell mediated response to gluten peptides in genetically susceptible individuals. 2) The adaptive immune system, particularly T cells, plays a crucial role in the pathogenesis by recognizing gluten peptides presented by HLA-DQ2 or HLA-DQ8 molecules. 3) This adaptive immune response leads to inflammation and damage to the intestinal mucosa. 4) In contrast, the innate immune system may also be activated, but it is not the primary driver of the disease. Summary: A: Correct - There is a strong genetic association with HLA alleles. B: Incorrect - Celiac disease is primarily driven by an adaptive immune response, not just features of autoimmunity. D: Incorrect - While gluten exposure triggers the immune response, the disease is not solely driven by gluten intake.

Correct Answer: D

Rationale: The correct answer is D. Resident dendritic cells in the gut produce anti-inflammatory cytokines like TGF-beta, promoting tolerance. The production of antimicrobial peptides in response to microbiota antigens helps maintain a balance with the microbiota. Induction of regulatory T cells (Tregs) also plays a crucial role in preventing excessive immune responses. Therefore, all of the choices (A, B, and C) contribute to maintaining gut immune system homeostasis by promoting tolerance and preventing inflammation.

Correct Answer: B

Rationale: Step 1: Secondary lymphoid tissues bring together antigen-presenting cells and lymphocytes, facilitating the activation of naïve lymphocytes. Step 2: Co-localization of these components is essential for efficient immune response initiation. Step 3: Maturation and expression of antigen receptors primarily occur in primary lymphoid tissues, not secondary. Summary: A: Incorrect. Maturation of lymphocytes occurs mainly in primary lymphoid tissues. C: Incorrect. Contact with self-antigens is necessary for self-tolerance and occurs in primary lymphoid tissues. D: Incorrect. Only choice B accurately describes the critical role of secondary lymphoid tissues.

Correct Answer: D

Rationale: Thymocytes undergoing positive selection express both CD4 and CD8 co-receptors along with a functional T-cell receptor (TCR). CD4 and CD8 help in recognizing MHC molecules, while TCR is crucial for antigen recognition. Positive selection ensures that thymocytes with a functional TCR that can interact with self-MHC molecules survive. Therefore, the correct answer is D, as thymocytes undergoing positive selection express TCR, CD4, and CD8. Choice A, TCR alone, is incorrect as CD4 and CD8 are also expressed. Choices B and C are incorrect as positive selection involves the expression of both CD4 and CD8, not just one or the other.