CD19, also known as B-lymphocyte antigen or Cluster of Differentiation 19, is a single-pass type I membrane protein that contains two Ig-like C2-type (immunoglobulin-like) domains. It is expressed on follicular dendritic cells and B cells, being present from the earliest recognizable B-lineage cells during development to B-cell blasts, but is lost upon maturation to plasma cells. CD19 primarily functions as a B cell co-receptor in conjunction with CD21 and CD81. Upon activation, the cytoplasmic tail of CD19 becomes phosphorylated, leading to binding by Src-family kinases and recruitment of PI-3 kinase. Mutations in the CD19 gene are associated with severe immunodeficiency syndromes, characterized by diminished antibody production. CD19 has been shown to interact with CD81, CD82, Complement receptor 2, and VAV2.