A patient with a central nervous system disorder is maintained on the drug methyldopa. Hemolytic anemia develops... This is MOST probably an example of

Correct Answer: B

Rationale: The correct answer is B: cytotoxic hypersensitivity. Methyldopa can induce an autoimmune response causing hemolytic anemia by forming antibodies against erythrocytes. This type of reaction is characterized by the destruction of red blood cells by the immune system. The process involves antibody binding to the surface of red blood cells leading to their destruction by immune cells. A: Atopic hypersensitivity is commonly associated with allergies like asthma, hay fever, and eczema, not hemolytic anemia. C: Immune-complex hypersensitivity involves the formation of antigen-antibody complexes leading to inflammation but not direct destruction of red blood cells. D: Cell-mediated hypersensitivity involves T cell activation and is more associated with delayed responses like contact dermatitis, not acute hemolytic anemia.

Correct Answer: D

Rationale: The correct answer is D: cytotoxic T cells. Cytotoxic T cells play a crucial role in graft and tumor rejection by directly targeting and killing foreign or abnormal cells. They recognize antigens presented on the surface of these cells and release cytotoxic molecules to destroy them. Non-complement-fixing antibodies (A) are involved in opsonization and neutralization but not the primary mediators of graft and tumor rejection. Phagocytic cells (B) such as macrophages can help in clearing cellular debris but are not as directly involved in targeting and destroying specific foreign or abnormal cells. Helper T cells (C) play a role in coordinating immune responses but are not the primary effectors in graft and tumor rejection.

Correct Answer: D

Rationale: The correct answer is D because T and B cells originate from hematopoietic stem cells in the bone marrow. If the patient has no detectable T or B cells, it indicates a defect in the stem cells originating in the bone marrow. A: The thymus is responsible for T cell maturation, not their origin. B: The bursal equivalent is specific to birds and not relevant in humans. C: T cell-B cell interaction occurs after the maturation of T and B cells, so a defect in this interaction would not lead to the absence of both cell types.

Correct Answer: A

Rationale: The correct answer is A. Class II MHC proteins are found on the surface of antigen-presenting cells such as macrophages, dendritic cells, and B cells, but not on T cells. They play a crucial role in presenting antigens to T cells. Choice B is correct as Class II MHC proteins exhibit high polymorphism, contributing to immune diversity. Choice C is correct as they are indeed involved in antigen presentation by macrophages. Choice D is correct as Class II MHC proteins have a binding site for CD4 proteins on helper T cells, facilitating antigen recognition. Therefore, the incorrect statement among the choices is A, as Class II MHC proteins are not found on the surface of T cells.