Which of the following statements is most correct?

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: 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.

Correct Answer: B

Rationale: The correct answer is B: NK cells. NK cells are part of the innate immune system and can directly kill virus-infected cells, providing early protection against viral infections. They do not require prior exposure to the virus to initiate their killing mechanism, making them crucial for early defense. Eosinophils (A), Neutrophils (C), and Mast cells (D) are not primarily involved in killing virus-infected cells. Eosinophils are more associated with parasitic infections, Neutrophils are primarily involved in bacterial infections, and Mast cells are involved in allergic reactions.

Correct Answer: A

Rationale: Step 1: Hypervariable regions are responsible for antigen binding specificity. Step 2: Complementary Determining Regions (CDRs) are the specific locations within the hypervariable regions. Step 3: As CDRs are part of the hypervariable regions, choice A is correct. Summary: Choices B, C, and D are incorrect because hypervariable regions are not in the constant region, are present in secreted antibodies, and are found in both heavy and light chains.