Patients with severely reduced C3 levels tend to have

Correct Answer: B

Rationale: The correct answer is B: increased numbers of severe bacterial infections. Severely reduced C3 levels indicate a deficiency in the complement system, which plays a key role in immune defense against bacteria. Without C3, the ability to opsonize bacteria and enhance phagocytosis is impaired, leading to an increased susceptibility to severe bacterial infections. The other choices are incorrect because low gamma globulin levels are associated with antibody deficiencies, severe viral infections are typically more common with T-cell deficiencies, and hemolytic anemia is linked to issues with red blood cell destruction rather than bacterial infections.

Correct Answer: D

Rationale: The correct answer is D because minor histocompatibility antigens can induce immune reactions leading to rejection in transplantation. These antigens are derived from genetic variations, not detected by antibodies and complement (A), controlled by genes outside the major histocompatibility complex (B), and are important in transplantation as they contribute to rejection responses (C).

Correct Answer: D

Rationale: The correct answer is D: class II MHC antigens. Antigen-presenting cells activate helper T cells by presenting antigens on their surfaces via class II MHC molecules. These molecules bind to the T cell receptor, triggering the immune response. IgE (choice A) is not involved in activating helper T cells. Gamma interferon (choice B) is a cytokine produced by T cells but is not directly involved in antigen presentation. Class I MHC antigens (choice C) present antigens to cytotoxic T cells, not helper T cells.

Correct Answer: B

Rationale: The correct answer is B: IgG antibody. In hemolytic disease of the newborn, maternal IgG antibodies against fetal Rh-positive red blood cells can cross the placenta and attack the fetal RBCs, leading to hemolysis. IgE is primarily involved in allergic reactions, not hemolytic disease. IgM antibodies are too large to cross the placenta efficiently. IgA is not typically involved in hemolytic diseases. Therefore, IgG is the most likely mediator of hemolytic disease of the newborn in cases of Rh blood group incompatibility.

Correct Answer: A

Rationale: The correct answer is A because the increasing hemagglutination inhibition titer from 10 on January 18 to 320 on February 20 indicates a rise in antibody levels, which is consistent with the patient being infected with influenza A virus. This suggests that the patient was ill with influenza A on January 15, as it takes time for antibody levels to rise in response to an infection. Choice B is incorrect because the highest titer was on February 20, which indicates peak antibody response, not the onset of illness. Choice C is incorrect as the increasing titer levels suggest an active infection. Choice D is incorrect as the titer results are indicative of an immune response to an infection, not an autoimmune disease.