Viral antigenic shift

Correct Answer: D

Rationale: Step 1: Viral antigenic shift involves major changes in the surface proteins of the virus, particularly the hemagglutinin (HA) protein. Step 2: These changes result in the emergence of new variants that are not recognized by preexisting antibodies. Step 3: This process is distinct from antigenic drift, which involves minor changes due to random mutations. Step 4: Choice D correctly states that viral antigenic shift refers to the appearance of new HA variants not recognized by preexisting antibodies. Summary: Choice A is incorrect because viral antigenic shift is not caused by multiple random mutations. Choice B is incorrect as antigenic shift involves major changes, not a single mutation. Choice C is incorrect as it affects surface proteins like HA, not just internal proteins.

Correct Answer: B

Rationale: The correct answer is B: Membrane-disrupting peptides. Defensins are antimicrobial peptides that disrupt bacterial cell membranes. This disrupts the integrity of the bacterial cell, leading to cell lysis and death. Choice A is incorrect because mast cells release histamines, not defensins. Choice C is incorrect as acute phase proteins are involved in the immune response to inflammation. Choice D is incorrect as opsonins are molecules that enhance phagocytosis of pathogens by immune cells.

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: 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: 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.