What would flow cytometry show in a patient with classic X-linked SCID?

In classic X-linked Severe Combined Immunodeficiency (SCID), there is a mutation in the IL2RG gene that primarily affects T cells and NK cells due to their developmental pathways being interconnected with the same gamma chain receptor used for signaling. This condition results in a significant deficiency of T cells, as they fail to develop properly in the thymus.

Flow cytometry in a patient with X-linked SCID typically reveals low or absent T cells because of this developmental blockade. Additionally, NK cells are also absent or severely reduced. However, B cell presence can be normal because the B cell development does not require T cell signals for their initial maturation; therefore, B cells may be present but functionally impaired without the help of T cells.

The profile observed in classic X-linked SCID – specifically low T cells, no NK cells, and normal B cells – is characteristic and aligns with the defective immune response resulting from this genetic condition. This distinct flow cytometry finding is critical for diagnosing X-linked SCID and differentiating it from other immunodeficiencies.