CD19 is a type I one-pass transmembrane protein and a biomarker for B cells and follicular dendritic cells. It has a single transmembrane domain, an extracellular N-terminus, and a cytoplasmic C-terminus. The highly conserved, 242 amino acid cytoplasmic domain includes several tyrosine residues, a small helical non-Ig domain with possible disulfide links separating the two extracellular C2 Ig-like domains.