An 8-year-old boy is brought to the pediatrician with fever, runny nose, and malaise. After examining the child, the pediatrician determines that he has a viral infection and does not require any specific treatment. The mother asks why an antibiotic is not necessary, and the physician explains the differences between bacterial and viral infections. The immune system is composed of both cellular and humoral components, which are able to mount an effective response against many types of viral and bacterial infections. A section of a normal lymph node is shown in the image below. The structures indicated by the arrows are most likely to contain cells undergoing which of the following processes?

Question

An 8-year-old boy is brought to the pediatrician with fever, runny nose, and malaise.  After examining the child, the pediatrician determines that he has a viral infection and does not require any specific treatment.  The mother asks why an antibiotic is not necessary, and the physician explains the differences between bacterial and viral infections.  The immune system is composed of both cellular and humoral components, which are able to mount an effective response against many types of viral and bacterial infections.  A section of a normal lymph node is shown in the image below.  The structures indicated by the arrows are most likely to contain cells undergoing which of the following processes?

Quizplus · Accepted Answer

B-cell precursors proliferate and mature in the bone marrow. Mature B-cells then leave the bone marrow and migrate to lymphoid organs and peripheral tissues, where they are exposed to antigens. On first exposure to a new antigen, a clone of B-cells becomes activated. Some activated B-cells differentiate into short-lived plasma cells that release antigen-specific IgM through a T-cell independent process. However, most activated B-cells migrate to lymphoid follicles located in the lymph node cortex where they form germinal centers (arrows) that are the site of B-cell proliferation during the immune response. A portion of these activated B-cells form long-lived memory cells that remain dormant in the lymph node until the next encounter with the same antigen, but the majority transform into antibody-secreting plasma cells. Isotype switching (from IgM to other types of immunoglobulins) also occurs in the germinal centers late in the primary response, providing activated B-cells the ability to produce antigen-specific antibodies of differing isotypes. Heavy chain constant regions are isotype-specific and distinguish the 5 isotypes (IgM, IgG, IgA, IgE, and IgD), while the variable regions are antigen-specific. Light chains are antigen-specific and do not determine isotype. Isotype switching first requires interaction of the CD40 receptor on activated B-cells with the CD40 ligand (CD154) expressed by activated T-cells. Afterward, isotype switching can occur through genetic rearrangement of the heavy chain constant regions. This process is modulated by T-cell cytokines such as IL-2, IL-4, IL-5, IL-6, and IFN-γ. After the primary immune response, subsequent encounters with the same antigen generate a predominantly IgG response (or IgA in the case of a mucosal response). (Choice B) Negative selection refers to the deletion of T-cell clones that strongly bind to self-MHC antigens. This process occurs in the fetal thymus and contributes to the development of tolerance to one's own antigens. (Choice C) Tolerance is immunologic unresponsiveness to self antigens. Central tolerance is acquired within the fetal thymus during negative selection. Peripheral tolerance develops by means of T-cell anergy, which is the functional inactivation of T-cells that are reactive to self antigens. (Choices D and E) Recombination of the V, D, and J regions of heavy chains and the V and J regions of light chains occurs via DNA rearrangement. After undergoing immunoglobulin gene rearrangement, each B-cell makes antibodies of a single specificity. An enormous variety of different immunoglobulin molecules can potentially be produced through rearrangement. Recombination of these regions occurs during B-cell maturation within the bone marrow. Later on, during the primary immune response, affinity maturation occurs in the germinal centers through the process of somatic hypermutation. Educational objective: The primary immune response to a new antigen initially results in plasma cells that only produce IgM. Isotype switching later occurs in the germinal centers of lymph nodes and requires interaction of the CD40 receptor on B-cells with the CD40 ligand (CD154) expressed by activated T-cells. IgG is the main serum immunoglobulin of the secondary response.

An 8-Year-Old Boy Is Brought to the Pediatrician with Fever