Avian Influenza has not so far caused a human pandemic because

Correct Answer: C

Rationale: The correct answer is C because avian influenza has not caused a human pandemic due to the fact that it recognizes a variant of sialic acid only found in deep tissues of humans. This unique binding preference prevents efficient human-to-human transmission, which is crucial for a pandemic to occur. Choice A is incorrect as avian flu can cause severe illness in humans. Choice B is incorrect because avian flu recognizing a variant of sialic acid found only on avian cells would not prevent a pandemic in humans. Choice D is incorrect as there is no specific annual vaccine for avian influenza.

Correct Answer: D

Rationale: The correct answer is D because a lower number of gut bacteria is not associated with obesity. In fact, research shows that an imbalance in gut bacteria, not a lower number, can contribute to obesity. Choice A is correct as there are indeed more bacteria in the gut than human cells in the body. Choice B is also correct as babies develop their commensal flora after birth. Choice C is correct, as studies on germ-free mice have shown the importance of microbiota in immune system development.

Correct Answer: B

Rationale: The correct answer is B because the 2009 H1N1 pandemic was caused by a newly assorted virus unrelated to existing human strains. This is atypical as typical pandemics are caused by animal strains. Option A is incorrect because the 2009 H1N1 pandemic did not emerge entirely as an assortment of human strains. Option C is incorrect as the 2009 H1N1 virus was not a variant of the 1918 H1N1 strain circulating in pigs. Option D is incorrect because the 2009 H1N1 pandemic did affect individuals across all age groups, including the very young.

Correct Answer: C

Rationale: The correct answer is C because a B cell epitope is not a part of the antibody molecule that contacts an antigen. B cell epitopes are specific regions on an antigen that bind to B cell receptors. Choice A is incorrect because denaturation can expose B cell epitopes. Choice B is incorrect because pathogens typically have multiple B cell epitopes. Choice D is incorrect because B cell epitopes can be found on various parts of pathogens, not just on the surface.

Correct Answer: C

Rationale: The correct answer is C because a given MHC molecule can bind only one peptide at a time. This is essential for the proper presentation of antigens to immune cells. MHC class I and class II molecules do not consist of two transmembrane protein chains (A), MHC class I molecules have one peptide-binding site while MHC class II molecules have one peptide-binding site (B), and MHC molecules on the cell surface typically have peptides bound to their peptide-binding sites, even in the absence of infection (D).