19.4 Immunodeficiency  (Page 2/6)

Selective iga deficiency

The most common inherited form of immunoglobulin deficiency is selective IgA deficiency , affecting about one in 800 people. Individuals with selective IgA deficiency produce normal levels of IgG and IgM, but are not able to produce secretory IgA . IgA deficiency predisposes these individuals to lung and gastrointestinal infections for which secretory IgA is normally an important defense mechanism. Infections in the lungs and gastrointestinal tract can involve a variety of pathogens, including H. influenzae , S. pneumoniae , Moraxella catarrhalis , S. aureus , Giardia lamblia , or pathogenic strains of Escherichia coli .

Severe combined immunodeficiency

Patients who suffer from severe combined immunodeficiency (SCID) have B-cell and T-cell defects that impair T-cell dependent antibody response s as well as cell-mediated immune response s. Patients with SCID also cannot develop immunological memory, so vaccine s provide them no protection, and live attenuated vaccines (e.g., for varicella-zoster, measles virus, rotavirus, poliovirus) can actually cause the infection they are intended to prevent. The most common form is X-linked SCID , which accounts for nearly 50% of all cases and occurs primarily in males. Patients with SCID are typically diagnosed within the first few months of life after developing severe, often life-threatening, opportunistic infection by Candida spp., Pneumocystis jirovecii , or pathogenic strains of E. coli .

Without treatment, babies with SCID do not typically survive infancy. In some cases, a bone marrow transplant may successfully correct the defects in lymphocyte development that lead to the SCID phenotype, by replacing the defective component. However, this treatment approach is not without risks, as demonstrated by the famous case of David Vetter (1971–1984), better known as “Bubble Boy” ( [link] ). Vetter, a patient with SCID who lived in a protective plastic bubble to prevent exposure to opportunistic microbes, received a bone marrow transplant from his sister. Because of a latent Epstein-Barr virus infection in her bone marrow, however, he developed mononucleosis and died of Burkitt lymphoma at the age of 12 years.

Secondary immunodeficiency

A secondary immunodeficiency occurs as a result an acquired impairment of function of B cells, T cells, or both. Secondary immunodeficiencies can be caused by:

• Systemic disorders such as diabetes mellitus , malnutrition , hepatitis , or HIV infection
• Immunosuppressive treatments such as cytotoxic chemotherapy , bone marrow ablation before transplantation, or radiation therapy
• Prolonged critical illness due to infection, surgery, or trauma in the very young, elderly, or hospitalized patients