An established method to evaluate T-cell function involves testing antibody production to vaccination with phiX174, a stable bacteriophage of E. Coli that is critical in demonstrating T-cell competence. Antibody titers after primary and secondary immunization correlate with abnormal CD4 cell help. Patients with functional B-cells that lack T-cell help show a characteristic failure to switch from IgM to IgG, making this assay essential in the evaluation of V(D)J recombination.