The percentage of developing thymocytes undergoing negative selection in the thymus is close to

Correct Answer: A

Rationale: The correct answer is A (90%). Negative selection in the thymus eliminates self-reactive T cells. The process ensures self-tolerance. The majority of developing thymocytes are removed during negative selection, making it close to 90%. Choices B, C, and D are incorrect because negative selection eliminates a high percentage of thymocytes, not a lower percentage as indicated in those options.

Correct Answer: D

Rationale: Step-by-step rationale: 1. NOD2 is a cytosolic receptor for bacteria, detecting bacterial molecules. 2. Mutations in NOD2 are found in 30% of patients with Crohn's disease. 3. NOD2 activation triggers NFκB signaling pathway. 4. Therefore, all the statements (A, B, C) are correct, making option D the correct answer. Other choices are incorrect as they do not encompass all the correct statements about NOD2.

Correct Answer: C

Rationale: The correct answer is C: Low antibiotic use. The hygiene hypothesis suggests that reduced exposure to infections, which can happen with low antibiotic use, can lead to an increased risk of immune diseases. Antibiotics can disrupt the balance of gut microbiota, which plays a crucial role in immune system development. A: Small family size may not necessarily protect against immune diseases as it does not directly relate to exposure to infections. B: Good sanitation can reduce the spread of infections but may not necessarily protect against immune diseases. D: Living in "westernized" countries is a broad factor and does not directly address the impact of low antibiotic use on immune diseases.

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: D

Rationale: The correct answer is D because all the statements in choices A, B, and C are valid reasons why we don't have a successful HIV vaccine. A: HIV has a high mutation rate, making it challenging to generate a protective memory response. B: Inducing mucosal immunity is difficult to block sexual transmission, a common route of HIV infection. C: HIV targets immune cells, which hinders the immune system's ability to mount an effective response. Therefore, all these factors combined pose significant challenges for creating an effective HIV vaccine.