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Classification of Primary Immunodeficiencies

For medical students3 min readUpdated 2026-10-10

Primary immunodeficiencies (PIDs) comprise a large group of inherited disorders characterized by impaired development or function of the immune system. Depending on the affected component, PIDs are divided into eight main groups, each with specific genetic causes, clinical presentations, and treatment approaches.

X-chromosomeMany severe forms of PIDs (SCID, Bruton disease) are associated with X-linked mutations.
Early onsetClinical manifestation of most immunodeficiencies occurs during the first weeks of life.
Baseline therapyLifelong antibiotics and immunoglobulin replacement therapy form the basis of treatment.
Radical approachHematopoietic stem cell transplantation can be used for SCID and complement defects.

Groups 1 and 2: Combined Immunodeficiencies and Antibody Deficiencies

Combined immunodeficiencies include severe combined immunodeficiency (SCID). The disease is most commonly caused by point mutations in the gene encoding the common cytokine receptor $\gamma$ chain for interleukins (IL-2, IL-4, IL-9) on the X chromosome. This leads to immunosuppression, characterized by decreased T-lymphocyte counts (CD4+, CD8+) and low immunoglobulin levels. Clinically, SCID manifests in the first weeks of life with skin and mucosal lesions, and severe pneumonia. The prognosis is extremely guarded due to the risk of sepsis. Treatment includes anti-infective and replacement therapy, with hematopoietic stem cell transplantation serving as a definitive treatment.

The second group comprises predominantly antibody deficiencies (B-cell defects). These arise from impaired B-lymphocyte differentiation or defective T-B cell interactions. A classic example is Bruton disease (X-linked agammaglobulinemia). A mutation in the Bruton tyrosine kinase (BTK) gene blocks B-cell maturation. Patients completely lack plasma cells, have profoundly decreased IgG levels, while IgA and IgM may be undetectable. T-cell counts remain normal. The mainstay of treatment is lifelong immunoglobulin replacement therapy.

Groups 3 and 4: Well-Defined Syndromes and Immune Dysregulation

Other well-defined immunodeficiency syndromes include conditions with specific clinical features, such as Job syndrome (hyper-IgE syndrome) associated with a mutation in the STAT3 gene (chromosome 4). The condition is characterized by early-onset eczema of the face and extremities, and recurrent purulent infections (most commonly staphylococcal or caused by Candida albicans). Laboratory findings show eosinophilia and a marked elevation of serum IgE. The prognosis is generally favorable, with symptomatic management.

The fourth group encompasses diseases of immune dysregulation, which lead to uncontrolled lymphoproliferation.

These conditions are managed with chemotherapy and glucocorticosteroids.

Groups 5 and 6: Phagocyte Defects and Innate Immunity Defects

Congenital defects of phagocytes involve impaired motility, chemotaxis, and microbicidal activity. This group includes:

  1. Chronic granulomatous disease.
  2. Cyclic neutropenia (ELANE gene defect, locus 19p13.3) — presents with a favorable prognosis despite periodic agranulocytosis.
  3. Congenital X-linked neutropenia (WAS gene defect, locus Xq11.22) — impaired promyelocyte maturation. There is a high frequency of bacterial infections and a risk of mortality from acute leukemia.

Therapy is based on lifelong antibiotic prophylaxis and gene therapy approaches.

The sixth group consists of defects of innate immunity. Pathologies of pattern recognition receptors or effector molecules lead to barrier tissue infections (furunculosis, paratonsillar abscesses). Examples include X-linked hypohidrotic ectodermal dysplasia with immunodeficiency (mutation in the NF-$\kappa$B essential modulator gene — IKBKG, locus Xq28) and human beta-defensin 1 deficiency (decreased protein expression due to point substitutions).

Groups 7 and 8: Autoinflammatory Disorders and Complement Deficiencies

The seventh group combines autoinflammatory disorders — immunodeficiencies characterized by systemic inflammation in the absence of a distinct infectious agent. A classic representative is hyper-IgD syndrome (mutation in the MVK gene, locus 12q24). It manifests with the classic triad: fever, leukocytosis, and hyperimmunoglobulinemia D.

The eighth group comprises complement system defects:

Management includes infection control, replacement therapy, and hematopoietic stem cell transplantation as a last resort.

Mnemonic

Many severe syndromes (Bruton disease, SCID, anhidrotic ectodermal dysplasia, and certain forms of neutropenia) are linked to the X chromosome.

Frequently asked questions

Which disorders are classified under congenital defects of phagocytes?

Congenital defects of phagocytes include the following disorders:

  • Chronic granulomatous disease — caused by impaired production of reactive oxygen species.
  • Cyclic neutropenia — characterized by periodic drops in neutrophil counts down to agranulocytosis.
  • Congenital X-linked neutropenia — accompanied by a sharp decrease in neutrophils due to impaired promyelocyte maturation.

Other innate immune cell defects include leukocyte adhesion deficiency (LAD), Chediak-Higashi syndrome, myeloperoxidase deficiency, and glucose-6-phosphate dehydrogenase deficiency.

How do the clinical presentations of early complement component deficiencies (C1q, C4) differ from late component deficiencies (C3, C9)?

The clinical presentation of complement deficiencies depends on the affected pathway level.

FeatureEarly Component Deficiency (C1q, C4)C3 or C9 Component Deficiency
Clinical PresentationDevelopment of autoimmune pathology, specifically an SLE-like syndromeRecurrent pyogenic infections of various types
PathogenesisGenetic defects in genes encoding early classical pathway proteinsImpaired antibacterial defense and humoral immunity
What clinical and laboratory findings characterize Job syndrome?

Job syndrome (hyper-IgE syndrome) is characterized by specific clinical and laboratory features:

  • Laboratory markers — markedly elevated serum IgE levels and peripheral blood eosinophilia.
  • Clinical presentation — recurrent purulent infections of the skin, subcutaneous tissue, soft tissues, and internal organs caused by staphylococci and fungi (Candida albicans).
  • Dermatological manifestations — eczema affecting the face and extensor surfaces of the extremities, debuting in the first weeks of life.
Which genes are associated with the development of severe combined immunodeficiency (SCID)?

Severe combined immunodeficiency (SCID) is associated with mutations in several genes:

  • Common cytokine receptor $\gamma$ chain gene — point mutations on the X chromosome (receptors for IL-2, IL-4, IL-7, IL-9, IL-15, IL-21).
  • ADA gene — leads to adenosine deaminase deficiency.
  • IL-7 receptor genes — cause defects in lymphopoiesis.
  • TBX1 gene — its deletion in DiGeorge syndrome variants disrupts thymic development.

Additionally, deficiency of the tyrosine kinase JAK3 presents with an identical SCID phenotype.

What treatment modalities are used for severe combined immunodeficiency?

Management of severe combined immunodeficiency (SCID) includes:

  • Baseline therapy — prophylactic anti-infective medications and immunoglobulin replacement therapy to protect against opportunistic pathogens.
  • Hematopoietic stem cell transplantation — a definitive treatment involving the transfer of allogeneic stem cells.
  • Gene therapy — an advanced approach where autologous bone marrow cells are transduced with the missing gene (e.g., ADA or the $\gamma$ chain gene) before reinfusion.
Which primary immunodeficiencies are associated with X-linked mutations?

The following primary immunodeficiencies are associated with X-linked inheritance:

  • Severe combined immunodeficiency (X-linked SCID).
  • X-linked agammaglobulinemia (Bruton disease).
  • X-linked lymphoproliferative disease (locus Xq25).
  • Congenital X-linked neutropenia (locus Xq11.22).
  • X-linked hypohidrotic ectodermal dysplasia (locus Xq28).
  • Chronic granulomatous disease (X-linked form, locus Xp21.1).
  • Wiskott-Aldrich syndrome.
  • Innate immunity defects — interleukin-1 receptor-associated kinase 4 (IRAK4) deficiency and others.
What immunophenotyping findings are typical for Bruton disease?

There is a dramatic reduction in IgG and circulating B lymphocytes, with a complete absence of plasma cells, IgA, and IgM. T-cell numbers and function remain normal.

What is the underlying mechanism of hyper-IgD syndrome?

It is an autoinflammatory disorder caused by an MVK gene mutation, characterized by systemic inflammation without an infectious trigger, presenting with recurrent fevers, leukocytosis, and elevated IgD levels.

What complications are typical of immune dysregulation diseases?

Major complications include chronic lymphoproliferation, malignancies (such as B-cell lymphoma), and autoimmune manifestations (e.g., cytopenias like autoimmune hemolytic anemia or neutropenia).

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