A protein that is a translation product of the human CD19 gene or a 1:1 ortholog thereof. It is composed of an N-terminal extracellular domain containing two Ig-like C2-type (immunoglobulin-like) domains, followed by a single-pass transmembrane segment and a cytoplasmic C-terminal tail. CD19 expression is restricted to members of the B cell lineage. It functions as a co-receptor for B-cell antigen receptor (BCR), regulating signal transduction. [ http://www.ncbi.nlm.nih.gov/pubmed/15778510 ]