Cell Types Affected in Severe Combined Immunodeficiency (SCID)

Primary Cell Types Affected in SCID

T cells are always affected in SCID, with very low or absent autologous T-cell counts and profoundly reduced proliferation to mitogens and antigens, which is the hallmark immunologic abnormality of SCID, according to the Journal of Allergy and Clinical Immunology 1, 2, 3
B cells are often functionally impaired, though numbers may be normal or reduced depending on the specific genetic defect, resulting in hypogammaglobulinemia due to both lack of T-cell help and intrinsic B-cell abnormalities, as reported by the Journal of Allergy and Clinical Immunology 1, 2
NK cells may be normal or reduced depending on the specific genetic defect, with some forms of SCID affecting T, B, and NK cells, while others primarily affect T cells or T and B cells, according to the Journal of Allergy and Clinical Immunology 4

Neutrophils are not primarily affected in classic SCID, except in reticular dysgenesis, an extremely rare form of SCID that also affects neutrophil development, as noted by the Journal of Allergy and Clinical Immunology 4, 5
Eosinophils are not primarily affected in SCID, according to the Journal of Allergy and Clinical Immunology 1, 2

The T-B-NK- phenotype is seen in adenosine deaminase (ADA) deficiency, which causes the most profound lymphopenia affecting all lymphocyte lineages, as reported by the Journal of Allergy and Clinical Immunology 4
The T-B+NK- phenotype is common in X-linked SCID (gamma chain deficiency) and JAK3 deficiency, characterized by the most B cells and fewest NK cells, according to the Journal of Allergy and Clinical Immunology 4
The T-B-NK+ phenotype is seen in RAG1/2 deficiencies and other defects in V(D)J recombination, as noted by the Journal of Allergy and Clinical Immunology 4
The T-B+NK+ phenotype is observed in IL-7 receptor deficiency and CD3 complex defects, according to the Journal of Allergy and Clinical Immunology 4, 5

Complete blood count typically reveals leukopenia and lymphopenia, as reported by the Journal of Allergy and Clinical Immunology 1, 2
Immunoglobulin levels show low or absent IgA and IgM with variable IgG levels, due to maternal transfer, according to the Journal of Allergy and Clinical Immunology 1, 2
Flow cytometry demonstrates severely reduced or absent T cells, with variable B and NK cell numbers, as noted by the Journal of Allergy and Clinical Immunology 6, 3
T-cell function tests show profoundly reduced proliferation to mitogens and antigens, according to the Journal of Allergy and Clinical Immunology 1, 2
Newborn screening using T-cell receptor excision circles (TRECs) can identify SCID early, as infants with SCID have very low or absent TRECs, as reported by the Journal of Allergy and Clinical Immunology 7, 8

The specific pattern of affected cells helps determine the underlying genetic defect and guides treatment approaches, according to the Journal of Allergy and Clinical Immunology 4
Hematopoietic stem cell transplantation (HSCT) is the definitive treatment for most forms of SCID, with better outcomes when performed early, as noted by the Journal of Allergy and Clinical Immunology 9

Very low or absent T-cell receptor excision circles (TRECs) are used in newborn screening programs to identify SCID, as recommended by the American Academy of Pediatrics 10
Poor specific antibody production is observed, with impaired responses to vaccines or natural infections, according to the American College of Allergy, Asthma, and Immunology 11

A suspicion of SCID should be considered an urgent clinical condition requiring immediate intervention, as stated by the American Academy of Pediatrics 12
Early diagnosis through laboratory testing is critical, as outcomes are significantly better when treatment (HSCT) is initiated before 3.5 months of age, with high strength of evidence from the American College of Allergy, Asthma, and Immunology 13
Premature infants may have transiently low TREC counts that increase over time, potentially leading to false-positive screening results, highlighting the need for careful interpretation of screening results, as noted by the American Academy of Pediatrics 13