Which of the following statements is incorrect?

Correct Answer: D

Rationale: The correct answer is D because the statement incorrectly implies that the bacterial type III secretion system is used by gut microbiota to invade intestinal epithelial cells in Crohn's disease. In reality, Crohn's disease is not directly caused by invasion of gut microbiota into intestinal cells; it is a complex autoimmune condition. The gut microbiota dysbiosis is associated with Crohn's disease, but invasion of epithelial cells is not the primary mechanism. Choices A, B, and C are correct: A highlights the altered microbiome in obesity, B discusses the use of fecal transplants for C. difficile infections, and C mentions Crohn's disease related to issues with gut microbiota tolerance.

Correct Answer: C

Rationale: The correct answer is C because the human microbiome does not secrete enzymes like lysozyme; instead, lysozyme is produced by the human body. A is correct as the microbiome can out-compete harmful bacteria. B is correct as the microbiome produces short chain fatty acids. D is correct as the microbiome helps in the development of the gut-associated immune system. Therefore, C is the only option that does not accurately describe a benefit from the human microbiome.

Correct Answer: B

Rationale: The correct answer is B because the major histocompatibility complex (MHC) plays a crucial role in the immune system by determining whether transplanted organs or cells will be accepted or rejected. MHC molecules present antigens to T cells, which then trigger an immune response leading to acceptance or rejection. Choice A is incorrect because T cell receptor gene rearrangement is not directly influenced by the MHC. Choice C is incorrect as MHC does not directly polarize T helper cells. Choice D is incorrect because MHC does not directly regulate the ratio of CD4+ and CD8+ T cells in an individual.

Correct Answer: C

Rationale: Passive immunization involves the transfer of pre-formed antibodies to confer immediate protection. Choice C is correct as it describes receiving serum containing antibodies from an immune individual. Choice A is incorrect because it describes active immunization. Choice B is incorrect as it refers to the primary immune response, which is part of active immunization. Choice D is incorrect as it describes the process of immunization itself, not passive immunization.

Correct Answer: A

Rationale: The correct answer is A: 1-C, 2-A, 3-B, 4-D. Immunoglobulins are antibodies with different functions: 1. IgG (C) - Main antibody in secondary immune response. 2. IgA (A) - Found in mucosal areas, neutralizes pathogens. 3. IgM (B) - First antibody produced in primary immune response. 4. IgE (D) - Involved in allergic reactions and defense against parasites. Other choices are incorrect as they mismatch the immunoglobulins with their functions.