Molecules that are considered members of the 'immunoglobulin superfamily':

Correct Answer: A

Rationale: The correct answer is A because molecules in the immunoglobulin superfamily typically contain one or more protein domains with a compact barrel structure. This structure is characteristic of immunoglobulin-like domains found in proteins involved in immune responses. Choice B is incorrect because not all members of the immunoglobulin superfamily are lymphocyte antigen receptors. Choice C is incorrect as these molecules are not necessarily highly unstable compared to other proteins. Therefore, the correct answer is A as it accurately describes a common structural feature of molecules in the immunoglobulin superfamily.

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.

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.