Humanized antigen-binding domains against CD19 and methods of use

The invention provides a humanized anti-CD19 antibody or antigen binding fragment thereof comprising a light chain variable (VL) region and a heavy chain variable (VH) region in which the humanized VL and VL regions are derived from the mouse anti-CD19 clone FMC63 antibody; the humanized VL and/or humanized VH region comprise one or more amino acid substitutions in the framework region. The humanized anti-CD19 antibody or antigen binding fragment may be part of a single chain variable fragment (scFv), a chimeric antigen receptor (CAR) or a T cell receptor (TCR). Other aspects of the invention relate to cells comprising the CAR or the TCR and their use in a T cell therapy.