| MHC EXPRESSION PATTERNS AND MHC RESTRICTED STIMULATION
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| MHC molecules can restrict stimulation to a limited number of cell types through differing properties of class I and class II molecules
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| Class I molecules are expressed on all nucleated cells of the body, whereas class II molecules are expressed on only a limited variety of cells. The latter includes cells whose primary role is to present antigen (antigen-presenting cells, APCs), including macrophages and the cortical cells of the lymph nodes, as well as B cells and activated T cells. In this way, the MHC molecules can restrict stimulation to a limited number of cell types but at the same time, using similar mechanisms, permit appropriate targeting of any cell in the body. Without such a control, the immune system could be triggered repeatedly for no good reason.
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| RECURRENT INFECTION IN AN IMMUNOCOMPROMISED PATIENT |
| A 2-year-old child presented with a history of recurrent Candida albicans and chest infections. Investigations reveal decreased number of neutrophils, IgG and IgA. Assessments of lymphocyte proliferative response show decreased expression of CD40 on T cells. A diagnosis of X-linked hyper IgM was made and a treatment with intravenous immunoglobulin was commenced. |
| Comment. T cell help is required for effective B-cell responses. Particular interactions are required for the switch of isotype from the IgM response that is typical of a primary antibody response, to the more mature IgG and or IgA isotypes seen in secondary antibody responses produced to subsequent challenges. CD40 L on the T cell is required to interact with the CD40 on B cells to achieve this. In its absence, antibody production is limited to IgM and the affected individual is immunocompromised owing to the lack of the other important isotypes so critical to the integrity of the immune response. The problem of infection more typically associated with cellular problems suggests the T-cell defect has functional consequences for this arm of the immune response. |
| As well as restricting input, the class I and II molecules also provide a differential mechanism for processing antigens that originate from within cells, e.g. viruses, and those that arise
from the extracellular environment, e.g. bacterial antigens. The different class MHC molecules also lead such antigens through different pathways to interact with the immune system, in particular with the T cells, on the basis that each will be better dealt with by differing effector systems: class I leads to CD8+ T-cytotoxic responses, and class II leads to antibody-mediated responses via CD4+ T- helper pathways.
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