Memory T cells

Correct Answer: C

Rationale: The correct answer is C because memory T cells have a lower threshold of activation than naïve T cells. This is because memory T cells have already been primed by previous exposure to antigens, making them more responsive upon re-exposure. This heightened sensitivity allows memory T cells to mount a faster and stronger immune response upon encountering the same antigen again. Choice A is incorrect because memory T cells primarily reside in peripheral tissues and not in secondary lymphoid tissues. Choice B is incorrect because memory T cells have encountered antigens before. Choice D is incorrect because memory T cells exist in a lower frequency compared to naïve T cells due to clonal expansion during the primary immune response.

Correct Answer: D

Rationale: The correct answer is D: Activation of TLRs. The earliest event in an inflammatory response is the activation of Toll-like receptors (TLRs) by recognizing pathogen-associated molecular patterns (PAMPs). This triggers a signaling cascade leading to the release of pro-inflammatory cytokines and chemokines, which then recruit neutrophils to the site of infection. Activation of adaptive immunity (choice C) occurs later in the immune response, as it involves the activation of T and B cells. While chemokines (choice A) are involved in recruiting immune cells, their release is a consequence of TLR activation. Neutrophils (choice B) are recruited after the release of chemokines.

Correct Answer: B

Rationale: The Black Death caused by Yersinia pestis is the correct answer because it led to immune adaptation in survivors. Yersinia pestis triggered the body to develop immunity. The Spanish flu pandemic, HIV/AIDS epidemic, and eradication of smallpox did not directly result in immune adaptation like the Black Death. The Spanish flu and HIV/AIDS were caused by viruses that did not lead to long-term immunity, and smallpox eradication was achieved through vaccinations, not natural immune adaptation.

Correct Answer: B

Rationale: Haematopoietic stem cells are responsible for producing immune cells. These stem cells differentiate into various types of blood cells, including white blood cells that play a crucial role in the immune system. They do not differentiate into lymph nodes (A), generate cytokines (C), or suppress pathogens directly (D). The correct answer (B) aligns with the primary function of haematopoietic stem cells in the production of immune cells.

Correct Answer: C

Rationale: The correct answer is C because a cell's response to a specific cytokine is determined by the specific cytokine receptors expressed on the cell. Cytokine receptors are proteins on the cell surface that bind to specific cytokines, triggering intracellular signaling pathways that lead to various cellular responses. The amount of antigen present (A) is related to the initiation of the immune response but not the cell's response to cytokines. The type of pathogen encountered (B) may influence the overall immune response but not the specific response to cytokines. The tissue location of the immune cell (D) can affect the types of cytokines present in the microenvironment but does not directly determine the cell's response to a specific cytokine.