Natural killer cells are

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: 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.

Correct Answer: A

Rationale: The correct answer is A: Streptococcal antigens induce antibodies cross-reactive with heart tissue. This is known as molecular mimicry, where antibodies produced against the streptococcal antigens also react with antigens present in heart tissue, leading to carditis. This mechanism is the basis for the development of rheumatic heart disease following a streptococcal infection. Choice B is incorrect because it does not explain the specific relationship between streptococcal infection and carditis. Choice C is incorrect as it describes a mechanism related to IgE and histamine release, which is not the primary mechanism involved in carditis. Choice D is incorrect as it focuses on neutrophils and proteases, which are not the primary mediators of carditis in the context of a streptococcal infection.

Correct Answer: D

Rationale: The correct answer is D because class I MHC proteins are primarily involved in presenting antigens to cytotoxic T cells to trigger an immune response against infected or abnormal cells. A is correct as class I MHC proteins are found on most cells. B is correct as they are indeed involved in T cell recognition. C is correct as they are expressed from both alleles. Choice D is incorrect because class I MHC proteins are not directly involved in the skin test response to Mycobacterium tuberculosis, which is primarily mediated by the immune response involving macrophages and T cells in the skin.