According to the hygiene hypothesis, which of the following factors protect us from developing immune diseases?

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 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.

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.