ARPC1B and severe combined immunodeficiency: Actin-related protein 2/3 complex subunit 1B (ARPC1B) deficiency is a recently described autosomal recessive disease characterized by combined immunodeficiency (CID), thrombocytopenia, and immune dysregulation having eczema, allergy, autoimmunity, and inflammatory diseases, in addition to an increased risk of severe infections as common features (1–6).