How does the secondary (memory) immune response differ from the primary immune response?

Correct Answer: D

Rationale: The correct answer is D because the secondary immune response is faster and stronger compared to the primary response. This is due to the presence of memory B and T cells from the primary response that can quickly recognize and respond to the antigen upon re-exposure. These memory cells allow for a more rapid and robust immune response, leading to quicker elimination of the pathogen. Choices A and B are incorrect because both primary and secondary responses involve the adaptive immune system and are antigen-specific. Choice C is incorrect as the secondary response is faster and more potent than the primary response.

Correct Answer: A

Rationale: Peyer's patches are lymphoid nodules located in the small intestine, specifically in the ileum. They play a crucial role in the immune response of the gut. The correct answer is A: Intestine because Peyer's patches are specifically found in the intestine. The other choices are incorrect because the spleen is a different organ involved in filtering blood, the lungs are primarily involved in respiration, and the skin is the body's largest organ responsible for protection and thermoregulation.

Correct Answer: B

Rationale: The correct answer is B: NOD2. Mutations in the NOD2 gene have been strongly associated with an increased risk of Crohn's disease. NOD2 is involved in the immune response to bacteria in the gut, and mutations in this gene can lead to dysregulation of the immune system, contributing to the development of Crohn's disease. A: TLR-4 is involved in the recognition of bacterial components but mutations in this gene are not strongly linked to Crohn's disease. C: NADPH oxidase is important for the production of reactive oxygen species in immune cells but mutations in this gene are not known to be a risk factor for Crohn's disease. D: IL-2 is a cytokine involved in immune regulation but mutations in this gene are not associated with an increased risk of Crohn's disease.

Correct Answer: D

Rationale: The correct answer is D because visceral leishmaniasis is indeed the most severe form of leishmaniasis. Visceral leishmaniasis affects internal organs and can be fatal if left untreated. A is incorrect because Leishmania is a protozoan parasite, not a bacterium. B is incorrect because leishmaniasis can affect individuals with normal immune systems, not just those with severe immunodeficiency. C is incorrect because a TH1 response is actually protective against severe leishmaniasis, not a TH2 response.

Correct Answer: B

Rationale: The correct answer is B: The type of pathogen it encounters. Dendritic cells produce different polarizing cytokines based on the type of pathogen they encounter. This is crucial for directing the differentiation of T helper (Th) subsets. Choice A is incorrect because the expression of MHC class I or II molecules by dendritic cells is important for antigen presentation to T cells, not for determining polarizing cytokines. Choice C is also incorrect because Fc receptors are involved in antibody-mediated immune responses, not in determining polarizing cytokines. Therefore, the correct answer is B as it directly influences the differentiation of Th subsets by providing appropriate polarizing cytokines based on the encountered pathogen.