Which of the following is a feature of antigen recognition by B cells?

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: 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.

Correct Answer: C

Rationale: Rationale: C is incorrect because naïve lymphocytes actually enter the lymph node through the afferent lymphatic vessel, not the efferent. A carries lymphocytes into the lymph node, and D is true since antigens enter through afferent vessels. B is correct, as naïve T cells are found in the paracortex.

Correct Answer: D

Rationale: The correct answer is D because neutralization by IgE along the mucosal surface of the gut is a major antibody-mediated mechanism against parasitic worms. IgE binds to antigens on the worm's surface, leading to the release of histamine and other mediators that help expel the worm. Neutrophil degranulation (A) is more associated with bacterial infections, opsonization (B) is mainly for phagocytosis of pathogens, and triggering physiological responses (C) is not a direct antibody-mediated defense against parasitic worms.