A naïve lymphocyte:

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: 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.

Correct Answer: C

Rationale: The correct answer is C, transfer of serum from someone who has survived an infection with Pathogen X. This provides passive immunity through the transfer of antibodies from the immune individual to the recipient, offering immediate protection. Choice A involves active immunity through vaccination, which requires the individual's immune response to produce antibodies. Choice B involves active infection, not passive immunity. Choice D mentions living amongst immunized individuals, which may confer herd immunity but does not provide passive immunity on an individual level.