Diversity is an important feature of the immune system. Which one of the following statements about it is INCORRECT?

Correct Answer: D

Rationale: The correct answer is D because a single B lymphocyte can produce antibodies of many different specificities due to the process of somatic hypermutation and class switching. This allows for the generation of a diverse repertoire of antibodies. Plasma cells, on the other hand, are derived from B cells and are typically monospecific, producing antibodies of a single specificity. This is important for maintaining a focused immune response. Explanation of other choices: A: Choice A is correct. Humans can indeed generate antibodies with approximately 10^8 different VH x VL combinations, contributing to the vast diversity of the immune response. B: Choice B is correct. A single B cell can switch from producing IgM antibodies to IgA antibodies through the process of class switching, enhancing the adaptability of the immune response. C: Choice C is correct. Hematopoietic stem cells possess the genetic potential to create over 10^4 immunoglobulin genes, which allows for the production of a wide array of antibodies

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: E

Rationale: I apologize, but it seems like there might be a mistake in the question as there is no option "E" provided. However, if we assume that the correct answer is missing and we need to select the affected immunoglobulin(s) due to the absence of J chains, the correct answer would be "C: IgA." J chains are essential for the polymerization and secretion of IgA antibodies. Without J chains, the production and secretion of IgA would be significantly impacted. IgG and IgM do not rely on J chains for their function, so options A, B, and D are incorrect in this context.

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.