MedGen

Reduced level of T cell receptor excision circle (TRECs) as measured by the TREC assay. Late in maturation, 70% of thymocytes that will ultimately express alpha/beta-T cell receptors form a circular DNA TREC from the excised TCRdelta gene that lies within the TCRalpha genetic locus. The circles are stable but do not increase following cell division and, therefore, become diluted as T cells proliferate. A quantitative polymerase chain reaction (PCR) reaction across the joint of the circular DNA provides the TREC copy number, a marker of newly-formed, antigenically-naïve thymic emigrant T cells. [from HPO]

MHC class II deficiency-4 (MHC2D4) is an autosomal recessive immunodeficiency characterized by the onset of recurrent, persistent infections from birth, resulting in failure to thrive. Infectious agents include bacteria, viruses, fungi, and protozoa, usually affecting the respiratory and gastrointestinal tract. Laboratory studies show decreased CD4+ T cells, hypogammaglobulinemia, an inverted CD4:CD8 ratio, and absence of MHC type II antigens (HLA-DR, -DQ, and DP) on the surface of antigen-presenting cells. Expression of MHC type I antigens may also be decreased (referred to as 'BLS type III'). Patients may die in infancy or early childhood, unless they undergo bone marrow transplantation, which can be curative, although complications are common. MHC2D4 may not be detected by newborn T-cell receptor excision circle (TREC) screening (summary by Mousavi Khorshidi et al., 2023). For a discussion of genetic heterogeneity of MHC class II deficiency, see MHC2D1 (209920). [from OMIM]

MHC class II deficiency-3 (MHC2D3) is a rare autosomal recessive immunodeficiency characterized by the onset of recurrent and persistent infections from birth. Infectious agents include bacteria, viruses, fungi, and protozoa, usually affecting the respiratory and gastrointestinal tract. Laboratory studies show decreased CD4+ T cells, hypogammaglobulinemia, an inverted CD4:CD8 ratio, and absence of MHC type II antigens (HLA-DR, -DQ, and -DP) on the surface of antigen-presenting cells. Most patients die in infancy or early childhood unless they undergo bone marrow transplantation, which can be curative, although complications are common. Rare patients may survive longer, even without bone marrow transplant. MHC class II deficiency may not be detected by newborn T-cell receptor excision circle (TREC) screening (summary by El Hawary et al., 2019; Mousavi Khorshidi et al., 2023). For a discussion of genetic heterogeneity of MHC class II deficiency, see MHC2D1 (209920). [from OMIM]