The gut immune system at homeostasis is 'tolerogenic'. This is achieved by:

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: 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.

Correct Answer: A

Rationale: Negative selection is the correct answer because it refers to the process that eliminates developing T cells with high affinity for self-peptide/MHC complexes to prevent autoimmune reactions. During negative selection, T cells that bind too strongly to self-antigens are deleted or rendered functionally inactive. This ensures that only T cells with moderate affinity for self-antigens are allowed to mature and participate in immune responses. Summary of other choices: B: Positive selection involves the selection of T cells that can recognize self-peptide/MHC complexes, promoting their maturation and survival. C: Peripheral tolerance refers to mechanisms that maintain self-tolerance in the mature immune system, not specifically related to eliminating self-reactive T cells during development. D: Lineage commitment is the process by which precursor cells differentiate into specific immune cell types and is not directly related to eliminating self-reactive T cells.

Correct Answer: A

Rationale: Step-by-step rationale: 1. In severe Covid-19 cases, there is usually a dysregulated immune response. 2. Type I Interferon plays a crucial role in early antiviral defense. 3. However, in severe cases, there is often a blunted or delayed response of type I Interferon. 4. This delayed response contributes to viral persistence and hyperinflammation. 5. Therefore, the prompt and sustained production of type I Interferon is not usually observed in severe Covid-19 cases. Summary: - Choice A is correct as the prompt and sustained production of type I Interferon is not typically observed in severe Covid-19 cases. - Choices B, C, and D are incorrect as autoantibodies, a delayed CD4 T cell response, and elevated levels of inflammatory cytokines are commonly observed in severe Covid-19 cases.

Correct Answer: B

Rationale: The correct answer is B because in immune response mechanism, clonal selection refers to the process where lymphocytes (T and B cells) proliferate and differentiate in response to a specific antigen. This process ensures that the immune system can effectively target and eliminate pathogens. Option A is incorrect because clonal selection does not refer to identifying a specific type of clone cell line in cell culture. Option C is incorrect because clonal selection does not involve cloning a specific gene by inserting it into bacteria using recombinant DNA technology. Option D is incorrect because clonal selection is not about selecting an appropriate cell type for cloning purposes. In summary, the correct answer, option B, best describes the process of clonal selection in the context of the immune response mechanism.