Reaction to poison ivy or poison oak is

Correct Answer: C

Rationale: The correct answer is C: a cell-mediated response. Poison ivy and poison oak reactions are primarily caused by contact with urushiol, an oil found in these plants. This oil triggers a cell-mediated immune response, involving T cells. T cells release cytokines that lead to inflammation and the characteristic rash. IgG and IgE responses are mainly involved in antibody-mediated immune responses, not in poison ivy reactions. Arthus reaction is a localized immune complex-mediated hypersensitivity reaction, not typically associated with poison ivy or poison oak.

Correct Answer: D

Rationale: The correct answer is D because the statement is false. Antigens do not preselect V and J segments for gene recombination. Instead, the process is random. V, D, and J segments do combine to encode the antigen-binding site, making choice C true. Choice A is true because there are many V region segments available for gene recombination. Choice B is also true as there are several J segments and several D segments available for recombination. Therefore, the correct answer is D because it inaccurately describes the process of gene segment recombination.

Correct Answer: B

Rationale: The correct answer is B (sample B) because the double-diffusion test in agar against antibody to bovine albumin is specific for detecting the presence of bovine albumin. Since horse blood does not contain bovine albumin, sample B must contain horse blood. Sample A does not contain horse blood as it would not react with the antibody to bovine albumin. Both samples (C) cannot contain horse blood because the test is specific for bovine albumin. Neither sample (D) is incorrect because sample B does contain horse blood.

Correct Answer: B

Rationale: The correct answer is B: Only virus X-infected cells of HLA-B27 type will be killed. This is because cytotoxic T cells recognize antigens presented by HLA molecules. In this case, the cytotoxic T cells from an HLA-B27 person will specifically target virus X-infected cells that display antigens on HLA-B27 molecules. Choice A is incorrect because not all virus X-infected cells will be killed, only the ones with specific HLA molecules. Choice C is incorrect as not all HLA-B27 cells will be killed, only the virus-infected ones. Choice D is incorrect as the question specifically mentions virus-infected cells, not all HLA-B27 cells.

Correct Answer: D

Rationale: The correct answer is D. Naïve Th cells do not produce both gamma interferon and interleukin-4 simultaneously. They differentiate into Th-1 or Th-2 cells based on signals received. Th-1 cells produce gamma interferon and promote cell-mediated immunity, while Th-2 cells produce interleukin-4 and -5 and promote antibody-mediated immunity. Both Th-1 and Th-2 cells have CD3 and CD4 proteins, but this is not the least accurate statement among the choices.