Due to its presence on all B cells, it is a biomarker for B lymphocyte development, lymphoma diagnosis and can be utilized as a target for leukemia immunotherapies.

[11] CD19 is a 95 kDa Type I transmembrane glycoprotein in the immunoglobulin superfamily (IgSF) with two extracellular C2-set Ig-like domains and a relatively large, 240 amino acid, cytoplasmic tail that is highly conserved among mammalian species.

The presence of a functional BCR is necessary during antigen-dependent differentiation and for continued survival in the peripheral immune system.

[23][24][25] Part of B cell differentiation is controlling c-MYC protein stability and steady-state levels through CD19, which acts as a PAX5 target and downstream effector of the PI3K-AKT-GSK3β axis.

Using a loss of function approach, researchers found reduced MYC levels in B cells of CD19 knockdown mice.

[23][26] On the cell surface, CD19 is the dominant signaling component of a multimolecular complex including CD21 (CR2, a complement receptor), TAPA-1 (a tetraspanin membrane protein), and CD225.

CD81, attached to CD19, is a part of the tetraspanin web, acts as a chaperone protein, and provides docking sites for molecules in various different signal transduction pathways.

[27] Recognition of an antigen by the complement system enables the CD19/CD21 complex and associated intracellular signaling molecules to crosslink to the BCR.

[11][27] CD19 has been shown to interact with: Mutations in CD19 are associated with severe immunodeficiency syndromes characterized by diminished antibody production.

[30] These mutations can lead to hypogammaglobulinaemia as a result of poor response to antigen and defective immunological memory.

[31] Researchers found changes in the constitution of B lymphocyte population and reduced amounts of switched memory B cells with high terminal differentiation potential in patients with Down syndrome.

[21][22] Additionally CD19 plays a role in modulating MHC Class II expression and signaling, which can be affected by mutations.

[13][34] However, it is now emerging that the protein plays an active role in driving the growth of these cancers, most intriguingly by stabilizing the concentrations of the MYC oncoprotein.