Isotype switching' of immunoglobulin classes by B cells involves

Correct Answer: B

Rationale: The correct answer is B because isotype switching involves the successive insertion of a single VH gene adjacent to different CH genes, leading to the production of antibodies of different classes. This process allows B cells to change the class of immunoglobulin they produce without altering the antigen specificity. Choice A is incorrect as it describes a mechanism different from isotype switching. Choice C is incorrect as it refers to the activation of genes on chromosome 6, which is not directly related to isotype switching. Choice D is incorrect as isotype switching specifically involves changing the heavy chain class of the immunoglobulin, not the light chain type.

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.

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.