Features include always present findings: Persistent EBV viremia, Decreased anti-CD3/28-induced T-cell proliferation, Generalized lymphadenopathy, and Enlarged spleen (splenomegaly); and common findings: Hypertriglyceridemia, Recurrent lower respiratory tract infections, Bronchiectasis, and Complete or near-complete absence of specific antibody response to tetanus vaccine and others.

TNFRSF9 function has not been fully characterized.

Immunodeficiency 109 with lymphoproliferation is caused by mutations in the TNFRSF9 gene on chromosome 1.

Genetic testing for TNFRSF9 is available. Testing is considered confirmatory for diagnosis.

Phenotype severity distribution: 4 always present features, 11 common features.

Estimated prevalence: <1 in 1,000,000 (VERY_RARE).