Objectives The aim was to further dissect the BCR and TLR signaling pathways involved in the establishement of central B-cell tolerance in humans. In detail, we aimed to analyze the role of CD19 in mediating central B-cell tolerance in humans. CD19 is a co-receptor expressed on B cells and is involved in the amplification of B-cell responses; its expression is decreased in patients with systemic lupus erythematosus (SLE), suggesting that proper CD19 expression may normally prevent autoimmunity. Additionally, CD19-deficient patients suffering from an antibody deficiency are also prone to develop systemic autoimmune diseases resembling SLE. Methods To test the function of the central B-cell tolerance checkpoint in humans, we analyzed by ELISA and immunofluorescence tests the reactivity of recombinant antibodies cloned from single transitional B cells from individuals carrying CD19 mutations. Additionally, we analyzed alterations in TLR and BCR signaling pathways in CD19-deficient human B cells using flow cytometry and immunoblotting.