Avian Influenza has not so far caused a human pandemic because

Correct Answer: C

Rationale: The correct answer is C because avian flu recognizing a variant of sialic acid only found deep in the lung tissue of humans, not in the upper airways explains why it has not caused a human pandemic. This specificity limits human-to-human transmission as the virus cannot efficiently attach in the upper airways where respiratory droplets are expelled. Choices A and B are incorrect because avian flu can cause severe infections in humans and recognizes a variant of sialic acid found in both birds and humans, not just avian cells. Choice D is incorrect as the presence of H5N1 in the annual trivalent vaccine does not prevent human pandemic potential.

Correct Answer: B

Rationale: The correct answer is B. MHC class I proteins bind to peptides from proteins made inside the cell and present them to CD8+ Tc cells. This statement is correct because MHC class I proteins are responsible for presenting endogenous peptides, derived from proteins synthesized within the cell, to CD8+ Tc cells for immune surveillance and response. Explanation for why other choices are incorrect: - A is incorrect because MHC class I proteins present endogenous peptides, not peptides from proteins taken up from outside the cell. - C is incorrect because MHC class I proteins present antigens to CD8+ Tc cells, not CD4+ Th cells. - D is incorrect because MHC class I proteins present peptides to CD8+ Tc cells, not CD4+ Th cells.

Correct Answer: D

Rationale: Memory T cells, compared to naïve T cells, take less time to become activated as they have previously encountered the antigen. They also have less need for costimulation during activation due to their enhanced sensitivity. Furthermore, memory T cells have a much longer life span, allowing for a more rapid and robust immune response upon re-exposure to the antigen. Therefore, all of the statements in choices A, B, and C are correct, making option D the correct answer.

Correct Answer: C

Rationale: The mucociliary escalator is a defense mechanism in the respiratory tract that traps and removes inhaled particles and pathogens by using cilia to move mucus out of the lungs. This helps to prevent infections and maintain lung health. Choices A and B are incorrect because the mucociliary escalator specifically functions in the respiratory tract, not the gut or skin. Choice D is incorrect because the mucociliary escalator does not protect all of the mentioned sites, only the respiratory tract.

Correct Answer: A

Rationale: Step 1: TLR4 is the receptor for lipopolysaccharide (LPS), a component of gram-negative bacteria. Step 2: The question states that normal mice injected with high doses of LPS die, indicating TLR4 activation. Step 3: Mutations in TLR4 can confer resistance to LPS-induced septic shock. Step 4: Therefore, the correct answer is A: TLR4, as it is directly involved in recognizing LPS and triggering septic shock. Summary: B: TLR5 does not specifically bind LPS. C: NOD1 and D: NOD2 are involved in intracellular recognition of bacterial components, not LPS.