Immunodeficiencies are disorders in which the immune system is unable to mount an effective protective response against pathogens, resulting in recurrent, severe, or unusual infections. They are classified as primary immunodeficiencies, caused by inherited genetic defects in immune system components, or secondary immunodeficiencies, acquired as a consequence of external factors that impair immune function.
Classification of Primary Immunodeficiencies
Primary immunodeficiencies (PIDs) are a heterogeneous group of over 450 genetic disorders caused by mutations in genes involved in immune system development and function. The International Union of Immunological Societies (IUIS) classifies PIDs into categories based on the predominant immune defect. Combined immunodeficiencies affecting both T and B cells include severe combined immunodeficiency (SCID), the most severe form, which presents in infancy with failure to thrive, chronic diarrhea, and life-threatening infections with opportunistic pathogens such as Pneumocystis jirovecii and cytomegalovirus. SCID is caused by mutations in genes essential for T cell development, including IL2RG (common gamma chain, X-linked SCID), ADA (adenosine deaminase deficiency), RAG1/RAG2 (defective V(D)J recombination), and JAK3. Without hematopoietic stem cell transplantation or gene therapy, SCID is fatal within the first year of life. Predominantly antibody deficiencies are the most common PIDs, accounting for approximately 50% of cases, and include common variable immunodeficiency (CVID), X-linked agammaglobulinemia (BTK mutation causing B cell development arrest at the pre-B cell stage), and hyper-IgM syndromes caused by defects in CD40L/CD40 interaction. Phagocyte defects include chronic granulomatous disease (CGD), caused by mutations in NADPH oxidase components (CYBB, CYBA, NCF1, NCF2) that prevent the respiratory burst needed to kill phagocytosed bacteria and fungi, leading to recurrent bacterial and fungal infections and granuloma formation. Complement deficiencies, though rarer, predispose to specific infections: deficiencies of early classical pathway components (C1, C4, C2) are associated with autoimmune diseases such as systemic lupus erythematosus, while deficiency of the membrane attack complex components (C5–C9) predisposes to recurrent Neisseria meningitidis infections.
Severe Combined Immunodeficiency
SCID represents the most profound form of primary immunodeficiency, characterized by absence or severe dysfunction of T cells with variable effects on B and NK cells. The clinical presentation typically occurs within the first three months of life with persistent diarrhea, failure to thrive, severe oral candidiasis, and Pneumocystis jirovecii pneumonia. The absence of a thymic shadow on chest radiography suggests SCID. The most common form is X-linked SCID due to IL2RG mutations, accounting for approximately 50% of cases. ADA-SCID, caused by adenosine deaminase deficiency, is associated with accumulation of toxic purine metabolites that are particularly harmful to lymphoid cells. Artemis (DCLRE1C) and ligase IV mutations cause SCID with radiation sensitivity. Newborn screening using T cell receptor excision circles (TRECs) as a biomarker of T cell production enables early diagnosis before infectious complications develop. Definitive treatment requires immune reconstitution through hematopoietic stem cell transplantation, ideally from an HLA-identical sibling, but haploidentical or matched unrelated donor transplantation can also be successful if performed early. Gene therapy using autologous stem cells corrected with a functional copy of the defective gene has achieved excellent results for X-linked SCID and ADA-SCID.
Common Variable Immunodeficiency
Common variable immunodeficiency (CVID) is the most common clinically significant primary immunodeficiency in adults, with an estimated prevalence of 1 in 25,000. CVID is characterized by low levels of serum immunoglobulins (IgG and IgA, with variable IgM), impaired antibody responses to vaccination, and onset typically after age 2 years, with a peak in adulthood. Affected individuals experience recurrent sinopulmonary infections with encapsulated bacteria such as Streptococcus pneumoniae and Haemophilus influenzae. Beyond infections, CVID is associated with autoimmune complications (cytopenias, autoimmune hemolytic anemia, immune thrombocytopenia), lymphoproliferation (splenomegaly, lymphadenopathy, granulomatous disease), and an increased risk of lymphoma and gastric cancer. The genetic cause is identified in only 10–30% of cases, with mutations in TNFRSF13B (TACI), TNFRSF13C (BAFFR), ICOS, and CD19 among the known variants. The mainstay of treatment is immunoglobulin replacement therapy administered intravenously or subcutaneously, which reduces infection frequency and improves quality of life. Antibiotic prophylaxis and management of autoimmune complications are additional components of care.
Phagocyte Immunodeficiencies
Chronic granulomatous disease (CGD) is the prototypical phagocyte immunodeficiency, affecting approximately 1 in 200,000 individuals. CGD is caused by defects in the NADPH oxidase enzyme complex that generates superoxide and other reactive oxygen species essential for killing catalase-positive bacteria and fungi in phagolysosomes. X-linked CGD (CYBB mutation, encoding gp91phox) accounts for approximately 65% of cases and is typically more severe than autosomal recessive forms (p47phox, p67phox, p22phox deficiencies). Patients with CGD develop recurrent infections with Staphylococcus aureus, Serratia marcescens, Burkholderia cepacia, Nocardia species, and Aspergillus species, and they develop inflammatory granulomas in the gastrointestinal and genitourinary tracts that can cause obstructive symptoms. Management includes prophylactic trimethoprim-sulfamethoxazole and itraconazole, interferon-γ therapy, and aggressive treatment of acute infections. Hematopoietic stem cell transplantation can cure CGD, and gene therapy approaches using lentiviral vectors are under investigation. Leukocyte adhesion deficiency (LAD) type I is caused by mutations in ITGB2 encoding CD18, the β₂ integrin common chain, impairing leukocyte adhesion to endothelium and migration to sites of infection, presenting with delayed separation of the umbilical cord, recurrent infections without pus formation, and severe periodontitis.
Secondary Immunodeficiencies
Secondary immunodeficiencies are far more common than primary forms and arise from external factors that impair immune function. Human immunodeficiency virus (HIV) infection causes progressive depletion of CD4+ T cells and is the most common severe secondary immunodeficiency globally, with an estimated 38 million people living with HIV. Without antiretroviral therapy, HIV infection progresses to acquired immunodeficiency syndrome (AIDS), characterized by opportunistic infections including Pneumocystis jirovecii pneumonia, Mycobacterium avium complex, cytomegalovirus retinitis, Cryptococcus neoformans meningitis, and Kaposi sarcoma caused by human herpesvirus 8. Malnutrition, particularly protein-energy malnutrition, zinc deficiency, and vitamin A deficiency, impairs T cell function, antibody production, and innate immune responses, and is the leading cause of secondary immunodeficiency in low-resource settings. Iatrogenic immunosuppression from chemotherapy, radiation therapy, immunosuppressive drugs used for autoimmune diseases (corticosteroids, calcineurin inhibitors, mycophenolate mofetil, TNF inhibitors), and immunomodulatory biologics (rituximab, alemtuzumab) predisposes to infections. Splenectomy or functional asplenia increases susceptibility to encapsulated bacteria, particularly Streptococcus pneumoniae and Neisseria meningitidis. Diabetes mellitus impairs neutrophil function and predisposes to infections with Staphylococcus aureus and Candida. Aging (immunosenescence) is associated with reduced T cell production, restricted T cell receptor diversity, impaired vaccine responses, and increased susceptibility to infections and reactivation of latent viruses including varicella-zoster virus.
Diagnosis and Management of Immunodeficiencies
The diagnosis of immunodeficiency begins with a clinical assessment of infection history, including the type, frequency, severity, and causative organisms of infections, as well as age of onset and family history. Sentinel infections suggesting immunodeficiency include recurrent pneumonia, sinusitis, or otitis media; severe infections with opportunistic organisms; failure to thrive; and infections with unusual pathogens. Initial laboratory evaluation includes complete blood count with differential, quantitative immunoglobulins (IgG, IgA, IgM), vaccine-specific antibody responses, lymphocyte subsets by flow cytometry (CD3+, CD4+, CD8+, CD19+, CD56+), and complement assays. Advanced testing for suspected PIDs includes lymphocyte proliferation assays, neutrophil function tests (dihydrorhodamine flow cytometry for CGD), and genetic testing through targeted gene panels or whole-exome sequencing. Management principles include antimicrobial prophylaxis for susceptible infections, immunoglobulin replacement therapy for antibody deficiencies, prompt and aggressive treatment of breakthrough infections, and definitive treatment through hematopoietic stem cell transplantation or gene therapy for severe combined immunodeficiencies and other eligible PIDs. Vaccination strategies differ in immunodeficient patients, with live vaccines contraindicated in severe T cell defects and antibody deficiencies, while inactivated and subunit vaccines are recommended where residual immune function allows protective responses.
