CD19 is a transmembrane glycoprotein that contains two extracellular immunoglobulin-like domains. It primarily acts as a B cell co-receptor in conjunction with CD21 and CD81. Upon activation, the cytoplasmic tail of CD19 becomes phosphorylated, which leads to binding by Src-family kinases and recruitment of PI-3 kinase. Mutations in CD19 are associated with severe immunodeficiency syndromes characterized by diminished antibody production. CD19 has also been implicated in autoimmune diseases and may be a useful treatment target.
