Virulence factors can be transferred from pathogenic to non-pathogenic bacterial strains that are part of the normal flora. An example of a commensal bacterial species that has turned into a pathogenic strain is:

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 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.

Correct Answer: B

Rationale: Step 1: Memory response produces antibodies of higher affinity due to affinity maturation. Step 2: Memory response involves class switching, leading to production of IgG, IgA, and IgE. Step 3: IgM is predominant in primary response, but memory response shifts towards other classes. Summary: Choice B is correct as memory response produces antibodies of higher affinity and different classes. Choices A and C are incorrect as memory response does not typically result in more IgM or lower affinity antibodies. Choice D is incorrect as only choice B is accurate.

Correct Answer: B

Rationale: Step 1: SCID is a genetic disorder affecting the immune system. Step 2: Inherited genetic mutations cause abnormalities in immune cells. Step 3: These mutations lead to severe deficiencies in immune function. Step 4: Choice B accurately reflects the genetic basis of SCID. Summary: Choice A is incorrect because SCID is not caused by HIV. Choice C is incorrect as it oversimplifies the immune deficiencies in SCID. Choice D is incorrect because SCID primarily affects immune cells, not stem cells.