Why do we constantly need new flu vaccines?

Correct Answer: D

Rationale: The correct answer is D. Choice A is correct because flu viruses mutate frequently due to their error-prone RNA polymerase, leading to new strains. Choice B is correct because the segmented nature of the viral genome allows for reassortment of different strains, creating completely new variants. Therefore, new vaccines are needed to target these evolving strains. Choice C is incorrect as the need for new vaccines is primarily driven by the genetic variability of the virus, not solely by the short-lived nature of antibodies. Thus, choices A and B together provide a comprehensive explanation for the constant need for new flu vaccines.

Correct Answer: C

Rationale: The correct answer is C: Transplantation. MHC proteins play a crucial role in transplant rejection by presenting antigens to T cells, initiating an immune response against foreign tissues. This discovery was significant in understanding transplant immunology. Choices A, B, and D are incorrect because innate immunity involves nonspecific defense mechanisms, allergies are mediated by IgE antibodies, and autoimmunity involves the immune system attacking self-antigens, none of which directly relate to the discovery of MHC proteins in transplantation studies.

Correct Answer: B

Rationale: The correct answer is B because TH2 cells play a crucial role in protecting against parasitic worms by secreting cytokines that promote eosinophil activation and antibody production. This response helps to expel the parasites from the body. Incorrect choices: A: TH1 cells are responsible for protection against intracellular bacteria. C: TH3 cells, not TH2 cells, are known for secreting anti-inflammatory cytokines. D: Activation of macrophages is primarily mediated by TH1 cells through IFN-gamma secretion.

Correct Answer: B

Rationale: The correct answer is B because lysozyme is not a type of defensin. Lysozyme is an enzyme present in secretions and tears, part of chemical innate barriers to infection, and capable of dissolving the cell wall of bacteria. Defensins are a different type of antimicrobial peptides that function by disrupting the microbial cell membrane. Therefore, B is the correct answer as it does not accurately describe lysozyme's function.

Correct Answer: D

Rationale: Pattern recognition receptors (PRRs) are a class of receptors that recognize specific molecular patterns associated with pathogens. They can be found in various locations within the body, including in soluble form in the blood (Choice A), in the cytosol of cells (Choice B), and on endosomal membranes (Choice C). Therefore, the correct answer is D, "All of the above are correct," as PRRs can indeed be located in all of these locations. PRRs play a crucial role in the innate immune response by detecting and responding to pathogens, making their presence in multiple cellular compartments essential for effective immune surveillance and defense.