Crohn's disease is

Correct Answer: D

Rationale: Step-by-step rationale for why answer D is correct: 1. Crohn's disease is characterized by chronic inflammation in the gastrointestinal tract. 2. It is associated with dysregulation of the immune response to gut flora. 3. Overgrowth of gut commensal flora can trigger and exacerbate inflammation in Crohn's disease. 4. Choices A, B, and C do not directly relate to the pathophysiology of Crohn's disease, making them incorrect.

Correct Answer: D

Rationale: The correct answer is D because coupling the polysaccharide subunit to a protein carrier enhances the immune response by converting the T-independent response to a T-dependent response. This allows for the induction of memory B cells and long-lasting immunity. Adjuvants (choice A) may enhance immune response but are not essential for polysaccharide-protein conjugate vaccines. Multiple booster injections (choice B) are not required for polysaccharide-protein conjugate vaccines as a single dose can induce a robust immune response. Administering orally (choice C) is not the only route for obtaining an immune response to polysaccharide subunit vaccines.

Correct Answer: D

Rationale: The correct answer is D, "All of the above." T cells help B cells undergo isotype switching by providing cytokines. They also help B cells produce antibodies against protein antigens by activating them. T cells provide signal 2 of activation to B cells through the interaction of CD40 on B cells with CD40L on T cells. Therefore, all of these functions are critical for the collaboration between T and B cells in the immune response.

Correct Answer: A

Rationale: The correct answer is A because the meeting of naïve T cells and dendritic cells is not facilitated by the presence of naïve B cells. Naïve T cells interact with dendritic cells in the paracortex of lymph nodes, not with naïve B cells. The movement of T cells along connective tissue networks aids in their interaction with dendritic cells, and this interaction may supply the T cell with activation Signal 1, which is essential for T cell activation. Overall, the incorrect choices (B, C, and D) are actually true statements regarding the meeting of naïve T cells and dendritic cells, making them incorrect options.

Correct Answer: B

Rationale: The correct answer is B because Ehrlich proposed that each cell producing antibodies would express several receptors, each specific for different antigens, whereas Burnet proposed that each single antibody-producing cell expresses only one type of receptor. This is the main difference between the two theories. Ehrlich's concept of multiple receptors per cell allows for a broader range of specificity and diversity in the immune response. The other choices are incorrect because they do not directly address the main difference between the two theories as outlined above. Choice A is incorrect because both Ehrlich and Burnet emphasized the importance of specificity, but they differed in their views on receptor diversity per cell. Choice C is incorrect because it focuses on the identity of soluble antibodies and receptors, which is not the main differentiator between the two theories. Choice D is incorrect as it mentions the release of soluble antigen receptors, which is not the main point of differentiation between Ehrlich and Burnet's theories.