A critical role of secondary lymphoid tissues is to:

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: 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.

Correct Answer: D

Rationale: The correct answer is D: Cell-mediated immunity. T lymphocytes are primarily involved in cell-mediated immunity, where they directly attack infected cells or pathogens. This type of immunity is crucial in fighting intracellular pathogens like viruses. Other choices are incorrect because: A: Passive immunity involves receiving pre-formed antibodies, not T lymphocytes. B: Humoral immunity involves B cells producing antibodies, not T lymphocytes. C: Acquired immunity is a general term encompassing both humoral and cell-mediated immunity, but T lymphocytes specifically play a key role in cell-mediated immunity.