Immunodeficiency Diseases Immunodeficiency can be divided into two categories: primary immunodeficiency diseases and secondary immunodeficiency diseases. Primary immunodeficiency diseases consist of disorders of the immune system in which the defect is intrinsic to the cells of the immune system. Secondary immunodeficiency diseases consist of disorders of the immune system in which the defect is induced by external factors, such as viruses, malignancy, and drugs. This section is clearly relevant to medical microbiology because the primary immunodeficiency diseases are usually identified first by the type of organism, duration, and frequency of infectious disease.

A. Primary Immunodeficiencies

 Primary immunodeficiencies are a heterogenous group of immune system disorders. Most of the primary immunodeficiencies are genetically determined and inherited as a single-gene defect. To date, more than 150 genetically based diseases have been identified. The genetic defect results in the loss of number or function of B cells, T cells, phagocytic cells, complement components, cytokines, or TLRs. Clearly, the loss of these functional elements leads to increased susceptibility to infections. One example is chronic granulomatous disease (CGD), which is an impairment of phagocytic cell function. Patients have normal levels of immunoglobulins, T and B cells, and phagocytic cells. However, the phagocytic cells do not kill microbes due to genetic defect in cytochrome b-558. This leads to a metabolic defect in the ability of phagocytic cells to produce peroxide and superoxide. The phagocytic defect can be identified by using the nitroblue tetrazolium (NBT) test. These cells are unable to efficiently kill some bacteria or fungi, such as, Staphylococcus, E coli, and Aspergillus spp. Unless treated, CGD is usually fatal within the first decade of life. IFN-γ has been shown to restore phagocytic function in these cells. Therefore, in most cases, administration of IFN-γ or bone marrow transplantation are the effective treatments. A second example is severe combined immunodeficiency (SCID). This syndrome is now known to be the final expression of several different genetic defects leading to defects in both B and T cell function. These individuals are susceptible to infection by virtually any microbe, and if untreated, they will die within the first year of life.

B. Secondary Immunodeficiencies

Secondary immunodeficiencies are a major predisposing cause of infection. Secondary immunodeficiency states are associated with infections, malignancies, and drugs.

C. Infections

 Infections can induce immunosuppression in the host. Historically, it is well known that patients infected with EBV and who present with mononucleosis have a depressed DTH skin test for TB and other antigens. This negative skin test indicates a depressed T cell response. Analysis of EBV replication has revealed a possible mechanism for this immunosuppression. Interestingly, the EBV genome codes for a human IL-10 analog. IL-10 is an immunosuppressive cytokine that inhibits Th1 cells from proliferating and producing cytokines, such as IFN-γ. This may account for the negative DTH skin test.

The most obvious example of a virus-induced immunodeficiency is HIV infection and the resulting disease, AIDS. HIV primarily infects CD4 T cells. This is possible because the virus uses the CD4 molecule itself as the virus receptor and the chemokine receptor, CCR5, as a coreceptor to enter the cell. HIV replication in CD4 T cells leads to a progressive loss of CD4 T cells and the development of AIDS. As a consequence of this infection, HIV patients develop multiple opportunistic infections. As noted earlier in this chapter, CD4 T cells are critically important for generating Th1, Th2, T h17, and T reg cell populations. These cell types are needed for a variety of immune reactions. These cells also provide help to B cells during antibody production and serve as a source of IL-2 and IFN-γ. Therefore, replication of a cytotoxic virus in this cell type is devastating to the immune response.

D. Malignancy

Selected leukemias, lymphomas, multiple myeloma, and other cancers can lead to immunodeficiency and increased infections. For example, patients with leukemia can have a deficiency in neutrophils, which results in loss of phagocytosis and increased infections with bacteria and fungi. Some tumors secrete high levels of TGF-β that can suppress a variety of responses, including Th1 responses.

 E. Drugs

Cytotoxic drugs used to treat cancer (eg, cisplatin), immunosuppressive drugs (eg, cyclosporine) that are used to manage transplant patients, and newer anticytokine (anti–TNF-α) drugs used to treat autoimmune diseases (eg, RA) can lead to increased risk of infection.

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مواضيع ذات صلة

The Immunoglobulin Domain and Variations

Hypersensitivity

Cytokines

Complement Deficiencies and Pathogen Evasion

Complement Pathways

T Cells

Antibody Responses

Immunoglobulin Genes and Generation of Diversity

Immunoglobulin Classes

Immunofluorescence

Chemiluminescence

Enzyme Immunoassay