Severe combined immunodeficiency (SCID) is a disease caused by:

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: 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.

Correct Answer: C

Rationale: The correct answer is C because innate immune response is not mediated by lymphocytes. Innate response involves physical barriers, such as skin and mucous membranes, and cells like macrophages and neutrophils. Lymphocytes are part of the adaptive immune response, which is characterized by specificity and memory. Choice A is correct as the innate response acts rapidly due to pre-existing mechanisms. Choice B is correct as adaptive response results in memory cells for quicker response upon re-exposure. Choice D is correct as adaptive response can recognize and respond to specific pathogens through antigen recognition.

Correct Answer: B

Rationale: The correct answer is B: Be highly immunogenic. A good vaccine should effectively stimulate the immune system to produce a strong immune response against the targeted pathogen while being safe for the individual. This is crucial for developing immunity without causing harm. Choice A (Be highly pathogenic) is incorrect because a vaccine should not cause disease or harm to the individual receiving it. Choice C (Induce herd immunity in a vaccinated individual) is also incorrect as herd immunity refers to protection of a population, not an individual. In summary, a good vaccine should be highly immunogenic to effectively trigger an immune response without causing harm, leading to protection against the targeted pathogen.

Correct Answer: B

Rationale: Explanation: An epitope is a specific region on an antigen that elicits the immune response and triggers the production of a specific antibody. Therefore, choice B is correct as it accurately defines an epitope. Choices A, C, and D are incorrect because they do not accurately describe what an epitope is. Choice A refers to the paratope, the region of the antibody that binds to the epitope. Choice C refers to a component of the antibody structure, and choice D refers to a specific region of the antibody structure.