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Common Variable Immunodeficiency is a heterogeneous group of disorders characterized by hypogammaglobulinemia, impaired antibody responses to infection (or vaccination), and increased susceptibility to infections.

Isolated IgA Deficiency is the Most common of all the primary immune deficiency diseases
IgA deficiency affects about 1 in 700 white individuals IgA is the major Ig in mucosal secretions = involved in airway and gastrointestinal defense

Hyper IgM = In a normal immune response to protein antigen, IgM antibodies are produced first, followed by the sequential elaboration of IgG, IgA, and IgE antibodies. As we discussed earlier in this chapter, the orderly appearance of different antibody types is called heavychain class (isotype) switching and is important for generating classes of antibody that can effectively activate complement and/or opsonize bacterial pathogens. The ability of IgMproducing B cells to turn on the transcription of genes that encode other Ig isotypes depends on certain cytokines, as well as contactmediated signals from CD4+ helper T cells. The contactdependent signals are provided by interaction between CD40 molecules on B cells and CD40L (also known as CD154), expressed on activated helper T cells. Patients with the hyperIgM syndrome produce normal (or even supranormal) levels of IgM antibodies to antigens but lack the ability to produce the IgG, IgA, or IgE isotypes; the underlying defect is an inability of T cells to induce Bcell isotype switching. The most common genetic abnormality is mutation of the gene encoding CD40L. This gene is located on the X chromosome; consequently, in approximately 70% of the cases, hyperIgM syndrome is Xlinked. In the remaining patients, the mutations affect CD40 or other molecules involved in class switching, notably an enzyme called activationinduced deaminase.

Thymic Hypoplasia: DiGeorge Syndrome results from a congenital defect in thymic development with deficient Tcell maturation. T cells are absent in the lymph nodes, spleen, and peripheral blood, and infants with this defect are extremely vulnerable to viral, fungal, and protozoal infections. Patients are also susceptible to infection with intracellular bacteria, because of defective Tcellmediated immunity. B cells and serum immunoglobulins are generally unaffected.