Primary immunodeficiencies are inherited disorders that impair one or more parts of the immune system. This lesson introduces their causes and shows how different defects produce characteristic patterns of infection.
A primary immunodeficiency (PI) results from an inherited defect in either nonspecific innate defenses or specific adaptive defenses. More than 250 primary immunodeficiencies have been identified. A defect may affect one immune component or several.
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What makes an immunodeficiency primary?
It is inherited and results from an absent or functionally defective component of innate or adaptive immune defense.
What is the immune defect in chronic granulomatous disease (CGD)?
A defect in the NADPH oxidase system prevents phagocytes such as neutrophils and macrophages from producing superoxide radicals in phagolysosomes, impairing microbial killing.
Why does X-linked agammaglobulinemia (XLA) cause a lack of antibodies?
A defective *BTK* gene means insufficient Bruton tyrosine kinase, halting B-cell maturation at the pre-B-cell stage before normal immunoglobulin production.
What is selective IgA deficiency?
An inherited immunoglobulin deficiency in which secretory IgA is absent or deficient despite normal IgG and IgM levels, increasing susceptibility to lung and gastrointestinal infections.
How does severe combined immunodeficiency (SCID) affect immune function?
Defects in both B cells and T cells impair antibody responses, including T-cell-dependent responses, and cell-mediated immunity. Patients also lack normal immunological memory and are vulnerable to severe opportunistic infections.
Quiz questions
What makes an immunodeficiency primary?Answer: An inherited defect in one or more innate or adaptive defenses.
How does NADPH oxidase dysfunction contribute to chronic granulomatous disease?Answer: It reduces phagocyte superoxide production, impairing the killing of microbes.
Why are people with X-linked agammaglobulinemia especially susceptible to pyogenic infections?Answer: B-cell maturation is blocked, leading to little or no antibody production against extracellular pathogens.
A person has normal IgG and IgM but cannot produce secretory IgA. Which infection pattern is most consistent with this defect?Answer: Greater susceptibility to lung and gastrointestinal infections because mucosal defenses are impaired.
Why can a live attenuated vaccine cause infection in a person with SCID?Answer: Severely impaired adaptive immunity may prevent the person from controlling the vaccine organism.
Based on Microbiology by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.