Immunodeficiency disorders
Also known as: immunodeficiencies, immune deficiency disorders
Immunodeficiency disorders are conditions in which part of the immune system is missing or dysfunctional, leaving the body vulnerable to recurrent, severe, or unusual infections. They are classified as primary (inherited) or secondary (acquired).
Immunodeficiency disorders arise when a component of the immune system — B cells, T cells, phagocytes, or complement proteins — is absent or fails to function. The hallmark is infection that is recurrent, unusually severe, or caused by organisms that rarely sicken healthy people.
Primary immunodeficiencies are inherited defects that typically present in infancy or childhood. Classic examples include X-linked agammaglobulinemia (absent B cells and antibodies), severe combined immunodeficiency (SCID, with defective B and T cells), DiGeorge syndrome (thymic aplasia causing T cell deficiency), and chronic granulomatous disease (phagocytes that cannot generate the oxidative burst). Secondary immunodeficiencies are acquired later in life from causes such as HIV infection, chemotherapy, immunosuppressive drugs, malnutrition, or malignancy.
The pattern of infection points to the missing component. B cell and antibody defects lead to recurrent infections with encapsulated bacteria; T cell defects predispose to viral, fungal, and opportunistic infections; phagocyte defects produce recurrent skin abscesses and infections with catalase-positive organisms; and complement deficiencies are associated with recurrent Neisseria infections. This mapping from clinical picture to defective immune arm is the central diagnostic skill.
The USMLE Step 1 tests immunodeficiency disorders heavily in its immunology section. High-yield questions present a child with a characteristic infection pattern and ask you to identify the disorder, the defective gene or cell type, and the expected laboratory findings.
Key takeaways
- Immunodeficiency disorders leave patients prone to recurrent, severe, or opportunistic infections.
- Primary immunodeficiencies are inherited (e.g., SCID, X-linked agammaglobulinemia, DiGeorge, chronic granulomatous disease); secondary forms are acquired (e.g., HIV, chemotherapy, immunosuppressants).
- B cell defects cause encapsulated bacterial infections; T cell defects cause viral, fungal, and opportunistic infections.
- Phagocyte defects lead to abscesses with catalase-positive organisms, and complement deficiencies to recurrent Neisseria infections.
- USMLE Step 1 questions test matching infection patterns to the defective immune component.