A positive tuberculin skin test (a delayed hypersensitivity reaction) indicates that

Correct Answer: B

Rationale: The correct answer is B because a positive tuberculin skin test indicates a cell-mediated immune response. This is because the test measures the response of T cells to the antigen, which is indicative of an active cellular immune response against Mycobacterium tuberculosis. Option A is incorrect as it refers to a humoral immune response involving antibodies produced by B cells. Option C is incorrect because the test specifically assesses T cell function, not both T and B cells. Option D is incorrect as it implies that only B cells are responsible for the positive skin test, which is not the case.

Correct Answer: B

Rationale: The correct answer is B because idiotypic determinants are unique to each antibody, so an antibody directed against the idiotypic determinants of a human IgG antibody would react with another antibody produced by the same plasma cell that produced the IgG, which would be an IgM antibody. Choice A is incorrect because idiotypic determinants are not located in the Fc region. Choices C and D are incorrect because idiotypic determinants are specific to the variable regions of antibodies, not the constant regions or the kappa/gamma chains.

Correct Answer: D

Rationale: The correct sequence for setting up a complement fixation test for antibodies is as follows: Ag + patient's serum + C (complement) / wait / + EA (erythrocyte antigen). This sequence ensures that the patient's serum containing antibodies is mixed with the antigen (Ag) first to allow for antibody-antigen binding. Then, complement (C) is added to the mix to activate the complement fixation process. Finally, erythrocyte antigen (EA) is added to observe any hemolysis indicating the presence of antibodies. This order is crucial for accurate test results. Incorrect choices: A: Incorrect sequence, as patient's serum should be mixed with Ag first. B: Incorrect sequence, as C should be added before EA. C: Incorrect sequence, as EA should be added last for hemolysis observation.

Correct Answer: A

Rationale: The high ratio of IgM to IgG antibodies indicates an acute infection, as IgM is produced initially and declines over time while IgG increases for long-term immunity. Choice A is correct because a high IgM to IgG ratio suggests a recent encounter with the virus, making it unlikely the patient has been previously exposed. Choices B, C, and D are incorrect because they do not address the specific antibody response pattern seen in acute viral infections.

Correct Answer: C

Rationale: The correct answer is C: removing mature T cells from the graft. Graft-versus-host disease (GVHD) occurs when the donor's T cells attack the recipient's tissues. Removing mature T cells from the graft can reduce the risk of GVHD while preserving the graft's ability to engraft and function. Matching complement components (choice A) may help in other transplant scenarios but not specifically in reducing GVHD. Administering alpha interferon (choice B) is not a standard method for preventing GVHD. Removing pre-B cells (choice D) does not target the cells responsible for GVHD.