The dark zone of germinal centers (GC) is composed of

Correct Answer: A

Rationale: The dark zone of germinal centers is composed of rapidly proliferating B cells. This is because during the dark zone phase of the germinal center reaction, B cells undergo rapid proliferation and somatic hypermutation to generate high-affinity antibodies. T cells and dendritic cells are typically found in the light zone of the germinal centers where B cells interact with them to receive signals for selection. Rapidly proliferating T cells are not typically found in the dark zone. Follicular dendritic cells are present in the light zone where they present antigens to B cells but not in the dark zone where B cell proliferation mainly occurs.

Correct Answer: C

Rationale: The correct answer is C: A strain of E. coli. "Hamburger disease" is commonly caused by E. coli O157:H7, a strain of Escherichia coli bacteria found in undercooked ground beef. This strain can lead to severe foodborne illness. Helicobacter pylori (Choice A) is associated with stomach ulcers, not hamburger disease. Salmonella typhimurium (Choice B) and Shigella flexneri (Choice D) are both known to cause food poisoning, but they are not typically linked to hamburger disease specifically. In this case, the unique association between E. coli O157:H7 and undercooked ground beef makes Choice C the correct answer.

Correct Answer: C

Rationale: Louis Pasteur's main contribution to immunology is the germ theory of disease and vaccine development. Firstly, Pasteur's germ theory revolutionized our understanding of infectious diseases by demonstrating that microorganisms are the cause. Secondly, he developed the concept of vaccination by creating vaccines for diseases such as rabies and anthrax, laying the foundation for modern immunology. The other choices are incorrect because: A: Pasteur worked on the rabies vaccine, not the smallpox vaccine. B: Phagocytosis was discovered by Metchnikoff, not Pasteur. D: Pasteur did not propose the theory of innate immunity.

Correct Answer: C

Rationale: Paul Ehrlich introduced the concept of discrimination between self and non-self through his "horror autotoxicus" theory. He proposed that the immune system can differentiate between harmful non-self antigens and self-antigens. Ehrlich's theory laid the foundation for understanding immune responses and the development of immunology. Metchnikoff focused on phagocytosis, Pasteur on microbiology, and Janeway on innate immunity, not specifically on self vs. non-self discrimination.

Correct Answer: B

Rationale: Neutrophils play a crucial role in the immune response by engulfing and destroying pathogens through a process called phagocytosis. This is essential for eliminating harmful invaders from the body. Neutrophils do not produce antibodies (choice A), activate lymphocytes (choice C), or synthesize cytokines (choice D), which are functions typically carried out by other immune cells like B cells and macrophages. Therefore, the correct answer is B as it directly aligns with the primary function of neutrophils in the immune response, which is to combat infections by destroying pathogens.