Antigen-driven responses by T cells from patients with CVID and normal subjects have been assessed. Low-density cells enriched for antigen-presenting dendritic cells were cultured with T cells using a 20-μl hanging drop system, T cells from all subgroups of CVID patients showed markedly reduced responses lo the recall antigens purified protein derivative (PPD) or tetanus toxoid. whereas responses by cells from patients with X-linked agammaglobulinaemia, used as a disease control, were in the normal range. However, primary allo-stimulation of CVID T cells was normal, CVID cells from two patients failed lo respond to stimulation with a neoantigen, an HIV env peptide. under conditions where normal T ceils did respond. These data illustrate a profound defect in antigen-stimulated T cell proliferation in vitro in all groups of CVID patients, but do not distinguish whether the defect is in the presenting cell or in the T lymphocyte. In vitro, germinal centre B cells are thought lo present antigen to primed T cells lo obtain essential signals (e.g. CD40 ligand and IL-2) for B cell survival and progression to immunoglobulin secretion. A failure of antigen-specific T cell function in vivo in CVID would thus not provide the primed T cells needed for B cell rescue, and could be the primary defect leading to the low immunoglobulin production in this condition.