A child stung by a bee experiences respiratory distress within minutes and lapses into unconsciousness. This reaction is probably mediated by

Correct Answer: A

Rationale: The correct answer is A: IgE antibody. IgE antibodies are involved in Type I hypersensitivity reactions, such as anaphylaxis caused by bee stings. IgE antibodies bind to mast cells and basophils, triggering the release of inflammatory mediators like histamine, causing respiratory distress and unconsciousness. IgG antibodies (choice B) are not typically involved in immediate hypersensitivity reactions. Sensitized T cells (choice C) are involved in delayed hypersensitivity reactions, not immediate ones like anaphylaxis. Complement (choice D) is part of the innate immune system and is not directly involved in IgE-mediated allergic reactions like anaphylaxis.

Correct Answer: D

Rationale: The correct answer is D because natural killer cells are a type of innate immune cell that can recognize and kill virus-infected cells without the need for prior sensitization. They are part of the body's first line of defense against viral infections. Choice A is incorrect because B cells are a type of adaptive immune cell involved in antibody production, not natural killer cells. Choice B is incorrect because cytotoxic T cells are also adaptive immune cells that require prior sensitization to target specific antigens. Choice C is incorrect because immunization typically boosts adaptive immune responses involving B and T cells, not natural killer cells.

Correct Answer: D

Rationale: The correct answer is D because in the determination of serum insulin levels by radioimmunoassay, we need isotope-labeled insulin (choice A) as a tracer for detection, anti-insulin antibody made in goats (choice B) to bind to insulin, and anti-goat gamma globulin made in rabbits (choice C) to detect the antibody bound to insulin. Isotope-labeled anti-insulin antibody made in goats (choice D) is not needed as it is not typically used in this assay. This choice would not contribute to the accurate measurement of serum insulin levels.

Correct Answer: B

Rationale: The correct answer is B: carcinoembryonic antigen (CEA). CEA is elevated in patients with progressive colon carcinoma and in normal fetuses. This is because CEA is produced during fetal development and is also present in certain cancers. It is not a viral antigen (A), as CEA is a protein produced by cells. Alpha-fetoprotein (C) is elevated in certain liver conditions and cancers, but not specifically in colon carcinoma. Heterophil (D) is a type of antibody produced in response to infection, not related to CEA levels in colon carcinoma.

Correct Answer: C

Rationale: The correct answer is C because the alternative complement pathway can be initiated independently of C3b fragments. It is activated by foreign surfaces directly, such as certain bacteria or viruses, without the need for prior activation of C3b. This is a key feature that distinguishes the alternative pathway from the classical pathway. Choices A, B, and D are incorrect because A is true - the alternative pathway can be triggered by infectious agents in the absence of antibodies, B is true - it does not require C1, C2, or C4, and D is false - the alternative pathway has a different terminal sequence of events compared to the classic pathway.