What is the primary function of white blood cells?

Correct Answer: C

Rationale: The primary function of white blood cells is to protect the body from foreign substances such as bacteria, viruses, and other pathogens. They do this by identifying and attacking these invaders to prevent infections and maintain overall health. White blood cells are a crucial part of the immune system and play a key role in defending the body against diseases. Clotting and sealing damaged blood vessels (choice A) is primarily done by platelets, not white blood cells. Transporting oxygen and carbon dioxide (choice B) is the function of red blood cells, not white blood cells. Producing hormones (choice D) is mainly performed by endocrine glands, not white blood cells.

Correct Answer: A

Rationale: The correct answer is A: E. coli 0157:H7. This strain of E. coli has acquired virulence factors that allow it to cause severe illness in humans, such as bloody diarrhea and kidney failure. The transformation of E. coli from a commensal to a pathogenic strain is well-documented. In contrast, choices B and C involve species that are inherently pathogenic and not commensal. Choice D is too broad and includes a wide range of bacterial species that do not necessarily originate from the normal flora. Therefore, E. coli 0157:H7 is the best example of a commensal bacterium that has turned pathogenic due to the acquisition of virulence factors.

Correct Answer: C

Rationale: The correct answer is C because NADPH oxidase is not constitutively expressed as a complex of 5 subunits in unstimulated cells. Instead, it is assembled and activated upon stimulation, such as during the respiratory burst following phagocytosis. Choice A is correct as NADPH oxidase is indeed involved in the respiratory burst. Choice B is correct as mutations in NADPH oxidase subunits lead to impaired pathogen killing and recurrent infections. Choice D is correct as NADPH oxidase controls the production of reactive oxygen species.

Correct Answer: C

Rationale: The correct answer is C because B cell receptors, known as antibodies, are Y-shaped proteins with two identical antigen-binding sites. This feature allows B cells to bind to specific antigens with high specificity. Each B cell receptor can bind to a unique antigen, leading to the initiation of an immune response. Choice A is incorrect because B cell receptors can be secreted as antibodies to target antigens outside the cell. Choice B is incorrect because B cell receptors recognize antigens directly, not in combination with MHC molecules. Choice D is incorrect because B cells primarily recognize specific antigens rather than general PAMPs associated with pathogens.

Correct Answer: B

Rationale: Correct Answer: B - Hematopoietic stem cells are not typically found within secondary lymphoid tissues because they are located in the bone marrow where they give rise to various blood cells. In contrast, secondary lymphoid tissues like lymph nodes and spleen primarily consist of mature immune cells and support immune responses. Naïve lymphocytes (A), dendritic cells (C), and antigens (D) are commonly found in secondary lymphoid tissues as they play crucial roles in the immune response.