If HLA-DR4 is considered a 'risk' allele for disease 'X', this means:

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: 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.

Correct Answer: A

Rationale: The correct answer is A: Natural Killer (NK) cells. NK cells are involved in antibody-dependent cell-mediated cytotoxicity by recognizing target cells coated with antibodies and inducing their death. NK cells play a crucial role in immune surveillance and defense against infected or malignant cells. Neutrophils, macrophages, and mast cells are not directly involved in antibody-dependent cell-mediated cytotoxicity. Neutrophils are primarily responsible for phagocytosis, macrophages are antigen-presenting cells, and mast cells are involved in allergic reactions.

Correct Answer: C

Rationale: The correct answer is C: Phagocytosis. Phagocytosis is a fundamental process that emerged early in evolution for cellular organisms to engulf and digest particles. This process is essential for basic survival and defense mechanisms. A: Antigen presentation and B: Antibody production are more advanced immune responses that occur in higher organisms and require a complex immune system. D: Thymic education is a process specific to vertebrates involving the maturation and selection of T cells, which is a more evolved mechanism compared to phagocytosis. Therefore, phagocytosis is the earliest process among the options listed as it is essential for basic cellular survival and predates the development of advanced immune responses.