In the late 1890s, 'cellularists' and 'humoralists' were intensively arguing about the key components driving the immune response. What was a crucial finding that led the humoralists to dominate the field in the following 50 years?

Correct Answer: B

Rationale: The correct answer is B, the discovery that cell-free serum could transfer immunity. This finding led humoralists to dominate because it supported the idea of circulating antibodies in the serum being responsible for immunity. This concept aligned with the humoralist theory, which focused on the role of antibodies in immune response. The ability of cell-free serum to transfer immunity provided direct evidence for humoralists' beliefs. Choice A (The demonstration of the Germ Theory of disease) is incorrect because while it was a significant advancement in understanding disease, it did not directly impact the argument between cellularists and humoralists regarding the immune response. Choice C (The discovery of the tuberculin reaction) is incorrect as it was more relevant to the understanding of the immune response to tuberculosis specifically, rather than the broader debate between cellularists and humoralists. Choice D (The determination of the antibody structure) is incorrect because while understanding antibody structure is crucial, it did not have as direct an impact on the dominance of

Correct Answer: A

Rationale: The correct answer is A: C5a. C5a is not part of the membrane attack complex (MAC). The MAC consists of C5b, C6, C7, C8, and multiple C9 molecules. C5a is a chemotactic factor that attracts immune cells to the site of infection, while C5b initiates the formation of the MAC. Choices B, C, and D (C5b, C6, C7) are all components of the MAC and play crucial roles in the formation and function of the complex.

Correct Answer: C

Rationale: Step-by-step rationale: 1. Naïve lymphocytes have antigen receptors but have not been activated by antigen. 2. Activation is necessary for their differentiation into effector cells. 3. Naïve lymphocytes are considered mature as they have completed development. 4. They circulate in secondary lymphoid tissues, not just primary lymphoid tissues. Summary: Choice C is correct because naïve lymphocytes have antigen receptors but have not been activated by antigen. Choices A, B, and D are incorrect because naïve lymphocytes do express antigen receptors, are considered mature, and can be found in both primary and secondary lymphoid tissues.

Correct Answer: D

Rationale: The correct answer is D, "All of the above." Autoimmune diseases have complex causes, involving genetic predisposition and environmental factors. Infections can trigger autoimmune responses, sunlight exposure can influence immune function, and an imbalance in microbiota composition can lead to dysregulated immune responses. Therefore, all three factors can contribute to an increased risk of autoimmune diseases. Choices A, B, and C individually are all associated with autoimmune diseases, making option D the correct choice.

Correct Answer: C

Rationale: The correct answer is C because having the HLA-DR4 allele increases the likelihood of developing disease X but does not guarantee it. Choice A is incorrect because not all individuals with HLA-DR4 will get disease X. Choice B is incorrect because not all individuals with disease X have the HLA-DR4 allele. Choice D is incorrect because it falsely states that all statements are true, which is not the case.