Immunophenotypic and Molecular Diagnostic Features of Severe Combined Immunodeficiency, Wiskott–Aldrich Syndrome, DOCK8 Deficiency, Activated PI3Kδ Syndrome, and Gray Platelet Syndrome: A Critical Narrative Review

Abstract

Severe combined immunodeficiency, Wiskott–Aldrich syndrome, dedicator of cytokinesis 8 (DOCK8) deficiency, activated phosphoinositide 3-kinase delta syndrome, and gray platelet syndrome are rare monogenic disorders in which diagnosis depends on the combined interpretation of multiparameter flow cytometric immunophenotyping and molecular genetic sequencing. Although individually reviewed within specialist literatures, these five conditions are rarely examined together despite sharing a common diagnostic architecture and, in several instances, overlapping clinical presentations that complicate differential diagnosis. This critical narrative review synthesises peer-reviewed evidence on the immunophenotypic and molecular features of the five disorders, evaluates the methodological strength of the underlying evidence base, and identifies points of diagnostic convergence and divergence. Severe combined immunodeficiency is characterised by profound T-lymphocyte deficiency detectable through population-based newborn screening and confirmed through defined lymphocyte-subset thresholds and a small core of causative genes. Wiskott–Aldrich syndrome and DOCK8 deficiency both present with combined cellular and humoral defects but are distinguished through protein-expression flow cytometry and eczema and platelet-size phenotypes respectively. Activated PI3Kδ syndrome exhibits a distinctive senescent T-lymphocyte and elevated immunoglobulin M phenotype linked to gain-of-function variants in the phosphoinositide 3-kinase pathway, and now has a licensed targeted therapy. Gray platelet syndrome, although a platelet rather than an immunodeficiency disorder, enters the same diagnostic pathway through macrothrombocytopenia and α-granule deficiency and offers an instructive counterpoint to Wiskott–Aldrich syndrome’s microthrombocytopenia. Across all five disorders, evidence quality is strongest where multicentre registries and consensus diagnostic criteria exist, and weaker where diagnosis rests on small, heterogeneous case series. The review identifies persistent gaps in the functional interpretation of variants of uncertain significance, in cross-disorder standardisation of flow cytometric protocols, and in long-term outcome data outside severe combined immunodeficiency. It concludes that immunophenotyping and molecular sequencing function as complementary rather than interchangeable tools whose relative diagnostic weight differs across these five disorders, with direct implications for laboratory diagnostic pathways and for the design of future genotype–phenotype studies.