The Fc regions of antibodies play a role in which of the following antibody effector functions:

Correct Answer: D

Rationale: The Fc regions of antibodies interact with various immune cells and molecules to mediate effector functions. - A: Degranulation of mast cells: Fc receptors on mast cells bind to antibody Fc regions, triggering degranulation. - B: ADCC: Fc regions bind to Fc receptors on immune cells, facilitating target cell killing. - C: Complement activation: Antibody binding to antigens can activate the complement system via the Fc region. Therefore, the correct answer is D because the Fc regions of antibodies are involved in all mentioned effector functions.

Correct Answer: A

Rationale: The correct answer is A: Louis Pasteur. He is considered to have created the first "attenuated" vaccine based on his work with the rabies vaccine in the 1880s. Pasteur's method involved weakening the virus to make it less harmful while still eliciting an immune response. Other choices are incorrect because: B: Robert Koch is known for his contributions to microbiology and germ theory, not for creating the first attenuated vaccine. C: Ilya Metchnikoff is known for his work on immunology and phagocytosis, not for developing the first attenuated vaccine. D: Adolf Von Behring is known for his research on diphtheria antitoxin, not for creating the first attenuated vaccine.

Correct Answer: A

Rationale: The correct answer is A: It is the causal agent of "hamburger disease." Salmonella typhimurium is not the causal agent of "hamburger disease." Instead, Escherichia coli O157:H7 is the primary causal agent of this particular foodborne illness. Salmonella typhimurium is commonly associated with causing salmonellosis. Therefore, choice A is incorrect. Choices B, C, and D are all true statements about Salmonella typhimurium. It is the second most common cause of food poisoning, it is able to induce its own uptake by manipulating the actin cytoskeleton of host cells, and it is able to survive inside the phagosomes by manipulating the host actin cytoskeleton.

Correct Answer: B

Rationale: Macrophages differentiate in tissues because they originate from monocytes which leave the bloodstream and mature into macrophages in various tissues. This process is essential for their diverse functions in immune response and tissue maintenance. Choices A, C, and D are incorrect because macrophages do not differentiate from blood neutrophils, the common lymphoid progenitor, or in the bone marrow.

Correct Answer: A

Rationale: The correct answer is A because naïve lymphocytes circulate through blood, enter lymph nodes (LN) via high endothelial venules (HEV), encounter antigens in LN, exit via efferent lymphatics, drain into the thoracic duct, and return to blood circulation. Choice B is incorrect because it skips the step of entering LN through HEV. Choice C is incorrect because it starts with inflammatory sites instead of blood, and it also lacks the step of entering LN through HEV. Choice D is incorrect as only option A correctly follows the sequential order of recirculation of naïve lymphocytes.