Congenital or acquired deficiency in the ability to fight infection:
  • Antibody production (B-cell)
  • Cellular immunity (T-cell)
  • Phagocytic dysfunction
  • Complement deficiency
  • Breach of skin/mucosal barriers


  • Congenital disorders
  • Immunosuppressive medications
  • Aging:
    • Immunosenescence
    • Poor circulation and wound healing
  • Chronic (lung, kidney, or heart) disease
  • HIV infection:
    • CD4 count determines susceptibility to pathogens
  • Diabetes:
    • Hyperglycemia impairs immune response
    • Vascular insufficiency
  • Malnutrition:
    • Poverty
    • Alcoholism and drug abuse
    • Eating disorders
  • Asplenia:
    • Functional asplenia (sickle cell disease) or surgical splenectomy increases risk of infection with encapsulated organisms
  • Organ transplantation:
    • Antirejection medications suppress immune response
    • Infections may be donor derived, recipient derived, or nosocomial
    • Increased risk of viral pathogens, such as cytomegalovirus, Epstein–Barr virus, and human herpes viruses
    • Time elapsed since transplantation is crucial, as different patterns of infection arise in early, intermediate, and late posttransplantation periods
  • Malignancy
  • Chemotherapy:
    • Increased risk of infection with pyogenic bacteria and fungi
    • Infection risk related to length and severity of neutropenia
  • Neutropenia:
    • Defined as absolute neutrophil count (ANC) <500/mm3 or <1,000/mm3 with an anticipated nadir of <500/mm3
    • In the U.S., gram-positive organisms are the leading etiology of infection
    • Gram-negative organisms are somewhat less common but often virulent
    • Polymicrobial infections are increasingly frequent
    • Anaerobic isolates remain relatively rare
    • The risk of fungal pathogens increases with prolonged neutropenia (>1 wk), prior use of broad-spectrum antibiotics, or intense chemotherapy

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