Which of the following statements about innate immunity is TRUE?

Correct Answer: C

Rationale: Step 1: Innate immunity is the body's first line of defense against pathogens. Step 2: Physical defenses include barriers like skin and mucous membranes. Step 3: Cellular defenses include immune cells like macrophages and natural killer cells. Step 4: These defenses are present from birth and do not require prior exposure to pathogens. Step 5: Choice C is true as innate immunity encompasses both physical and cellular defenses, making it the correct answer. Other choices are incorrect: A: Innate immunity does play a role in stimulating adaptive immunity through antigen presentation. B: It does not require prior exposure to function, as it is a rapid and non-specific response. D: Innate immunity is not antigen-specific; adaptive immunity is responsible for antigen-specific responses.

Correct Answer: A

Rationale: The correct answer is A because memory function in adaptive immunity allows for rapid and stronger responses to previously encountered antigens. This is achieved through the presence of memory cells, such as memory B and T cells, which can quickly recognize and respond to specific pathogens upon re-exposure. This leads to a faster and more efficient immune response, providing better protection against future infections. Choice B is incorrect because direct killing of pathogens without prior activation is a function of innate immunity, not adaptive immunity. Choice C is incorrect as enhanced cytokine secretion is not the primary advantage of memory function. Choice D is incorrect because while memory function provides protection against previously encountered antigens, it does not confer resistance to all infections without exposure.

Correct Answer: B

Rationale: After clonal selection and expansion, B cells differentiate into plasma cells to produce antibodies. This is crucial for the immune response as plasma cells are specialized in antibody production. B cells do not produce cytokines (choice A), initiate phagocytosis (choice C), or present antigens to T cells (choice D) post-clonal selection. Plasma cell differentiation is the key outcome of the B cell activation process, enabling the body to mount an effective immune response against pathogens.

Correct Answer: B

Rationale: The correct answer is B because adaptive immunity is characterized by antigen-specific responses and memory. This means that it can recognize and target specific antigens upon exposure and develop long-lasting immunological memory. Choice A is incorrect because physical barriers like skin are part of innate immunity, not adaptive immunity. Choice C is incorrect as adaptive immunity responds to specific antigens, not all antigens non-specifically. Choice D is incorrect because adaptive immunity typically takes longer to respond compared to innate immunity due to the need for activation and clonal expansion of specific immune cells.

Correct Answer: C

Rationale: The correct answer is C: Mucosal immunity. IgA plays a crucial role in protecting mucosal surfaces, such as the lining of the gut and respiratory tract, from pathogens. It helps prevent infections by neutralizing and blocking the entry of harmful substances. IgA is not involved in allergy mediation (choice A), placental transfer (choice B), or primary response (choice D), as those functions are typically associated with other types of antibodies. IgA's specificity for mucosal surfaces makes it uniquely suited for providing defense at these vulnerable sites.