Which of the following complement proteins is not part of the membrane attack complex?

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: 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.

Correct Answer: D

Rationale: The correct answer is D: Production of IgE antibodies. A TH2 response is characterized by the production of IgE antibodies, which are essential in allergic reactions. IgM antibodies (A) are produced during the primary immune response. Activation of CTL (B) is associated with a TH1 response. Neutrophils (C) are primarily involved in the innate immune response and are not specific to TH2 responses. In summary, the production of IgE antibodies is a defining characteristic of a TH2 response, distinguishing it from other immune responses.

Correct Answer: D

Rationale: IgA can be found in the intestinal lumen, lamina propria, and vesicles of enterocytes. IgA is produced by plasma cells in the lamina propria and transported across the epithelial cells into the intestinal lumen where it provides mucosal immunity. It can also be found in vesicles of enterocytes before secretion. Therefore, the correct answer is D as IgA is present in all of the mentioned locations. Choices A, B, and C are incorrect individually as they only represent specific locations where IgA can be found, not all the locations where IgA is present in the gastrointestinal tract.