Primary Immunodeficiencies: Combined and B-Cell Disorders

This topic covers inborn errors of immunity that impair adaptive immunity through defective antibody production or combined failure of both T-cell and B-cell function. The spectrum runs from severe combined immunodeficiency (SCID), a paediatric emergency in which both cellular and humoral arms are absent, to predominantly antibody deficiencies such as agammaglobulinaemia and common variable immunodeficiency that present with recurrent bacterial infection.

Definition

Combined immunodeficiencies are inborn errors in which both T-cell and B-cell adaptive immunity are defective; in severe combined immunodeficiency (SCID) the defect is profound and early-onset. B-cell (antibody/humoral) immunodeficiencies are inborn errors that primarily impair immunoglobulin production while T-cell numbers may be preserved, as in X-linked agammaglobulinaemia and common variable immunodeficiency.

Scope

The entry surveys the combined and B-cell categories of the international classification of inborn errors of immunity: severe and combined immunodeficiencies, agammaglobulinaemia, common variable immunodeficiency, selective IgA deficiency, and related antibody defects. It frames how these disorders are classified by the affected lymphocyte lineage and the infection patterns they produce, as a reference rather than a diagnostic or treatment protocol.

Core questions

• Is the defect purely humoral (antibody) or combined (both T and B cells)?
• What infection pattern distinguishes antibody deficiency from combined immunodeficiency?
• Why is SCID considered a paediatric emergency requiring early recognition?

Key concepts

• Severe combined immunodeficiency (SCID)
• T-B-NK lineage typing of SCID
• X-linked agammaglobulinaemia (Bruton)
• Common variable immunodeficiency (CVID)
• Selective IgA deficiency
• Hypogammaglobulinaemia
• Class-switch recombination defects (hyper-IgM)
• Newborn screening by TREC assay
• Recurrent pyogenic bacterial infection

Mechanisms

In combined immunodeficiencies, defects in the genes governing lymphocyte development, signalling, or survival prevent functional T cells from arising, and because B-cell help depends on T cells, antibody responses fail as well; in SCID the result is an infant with no effective adaptive immunity, vulnerable to viral, fungal, and opportunistic infection from the first months of life (Fischer, 2002). The classification subdivides SCID by which lineages are absent (for example T-B-NK profiles), reflecting the underlying molecular defect (Tangye, 2022). In predominantly antibody deficiencies, the block lies in B-cell development or terminal differentiation: X-linked agammaglobulinaemia abolishes mature B cells and immunoglobulin, whereas common variable immunodeficiency is a heterogeneous failure of antibody production that often also carries autoimmune and inflammatory complications (Cunningham-Rundles, 2012). The shared consequence of antibody failure is susceptibility to encapsulated and other pyogenic bacteria (Notarangelo, 2010).

Clinical relevance

Recognising the difference between isolated antibody deficiency and combined immunodeficiency frames how the recurrent-infection patient is conceptualised, and SCID is the archetypal condition for which newborn screening was developed. As a reference topic it explains how these disorders are classified and associated with characteristic infections; it is educational and not a substitute for clinical evaluation or management (Bonilla, 2015).

Epidemiology

Predominantly antibody deficiencies are the most common category of inborn errors of immunity, with selective IgA deficiency the most frequent and common variable immunodeficiency the most frequent symptomatic antibody deficiency. SCID is rarer but, untreated, is usually fatal in infancy, which is why population newborn screening using the T-cell receptor excision circle (TREC) assay has been adopted in many regions (Bonilla, 2015; Notarangelo, 2010).

Evidence & guidelines

The categories and their molecular bases are codified in the IUIS classification of inborn errors of immunity (Tangye, 2022), and a joint-society practice parameter summarises how primary immunodeficiencies are approached (Bonilla, 2015). Reviews address SCID and its treatment (Fischer, 2002) and the clinical heterogeneity of common variable immunodeficiency (Cunningham-Rundles, 2012).

History

Bruton's 1952 description of agammaglobulinaemia in a boy with recurrent infection established that a single immune component could be congenitally absent, and subsequent decades defined SCID and the antibody-deficiency syndromes at the cellular and molecular level. SCID became the first human disease treated by haematopoietic stem-cell transplantation and an early target of gene therapy, while population newborn screening later transformed its early detection (Fischer, 2002; Tangye, 2022).

Key figures

• Alain Fischer
• Charlotte Cunningham-Rundles
• Luigi Notarangelo
• Stuart Tangye
• Ogden Bruton

Seminal works

• tangye-2022
• fischer-2002
• cunningham-rundles-2012
• notarangelo-2010

Frequently asked questions

What distinguishes a combined immunodeficiency from a pure antibody deficiency?

In a combined immunodeficiency both T-cell and B-cell function are impaired, so the patient is vulnerable to viral, fungal, and opportunistic infections as well as bacterial ones; in a pure antibody (B-cell) deficiency T cells are largely preserved and the main problem is recurrent pyogenic bacterial infection.

Why is severe combined immunodeficiency screened for in newborns?

Because untreated SCID is typically fatal within the first year of life, and early detection before infections occur allows definitive treatment to be planned; the TREC assay on newborn blood spots detects the absence of newly formed T cells that characterises SCID.