"tective 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.\n\nA secondary immunodeficiency occurs as a result of an acquired impairment in the 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; and prolonged critical illness due to infection, surgery, or trauma in the very young, elderly, or hospitalized patients. Unlike primary immunodeficiencies, which have a genetic basis, secondary immunodeficiencies are often reversible if the underlying cause is resolved. Patients with secondary immunodeficiencies develop an increased susceptibility to an otherwise benign infection by opportunistic pathogens such as Candida spp., P. jirovecii, and Cryptosporidium. HIV infection and the associated acquired immunodeficiency syndrome (AIDS) are the best-known secondary immunodeficiencies. AIDS is characterized by profound CD4 T-cell lymphopenia (decrease in lymphocytes). The decrease in CD4 T cells is the result of various mechanisms, including HIV-induced pyroptosis (a type of apoptosis that stimulates an inflammatory response), viral cytopathic effect, and cytotoxicity to HIV-infected cells. The most common cause of secondary immunodeficiency worldwide is severe malnutrition, which affects both innate and adaptive immunity. More research and information are needed for the more common causes of secondary immunodeficiency; however, the number of new discoveries in AIDS research far exceeds that of any other single cause of secondary immunodeficiency. AIDS research has paid off extremely well in terms of discoveries and treatments; increased research into the most common cause of immunodeficiency, malnutrition, would likely be as beneficial.\n\nTable 1.22 summarizes primary and secondary immunodeficiencies, their effects on immune function, and typical outcomes. Primary immunodeficiencies include chronic granulomatous disease, which causes impaired killing of bacteria within the phagolysosome of neutrophils and macrophages and results in chronic infections and granulomas; selective IgA deficiency, which causes an inability to produce secretory IgA and a predisposition to lung and gastrointestinal infections; severe combined immunodeficiency disease (SCID), which causes deficient humoral and cell-mediated immune responses and early development of severe and life-threatening opportunistic infections; and X-linked agammaglobulinemia, which causes flawed differentiation of B cells and absence of specific antibodies and results in recurrent infections almost exclusively due to pathogens that cause pyogenic infections. Secondary immunodeficiencies include immunosuppressive therapies such as chemotherapy and radiotherapy, which impair humoral and/or cell-mediated immune responses and result in opportunistic infections and rare cancers; malnutrition, which impairs humoral and/or cell-mediated immune responses and results in opportunistic infections and rare cancers; and viral infection such as HIV, which impairs cell-mediated immune responses due to CD4 T-cell lymphopenia and results in opportunistic infections and rare cancers."