B-lymphocytes are essential in specific immune responses. In this process, B lymphocytes control the antibody-based defence response of the body.
Via immunoglobulins (Ig) on the cell surface, B lymphocytes can recognize specific antigens. These antigens are proteins foreign to the body.
In addition to immunoglobulins, B lymphocytes have other markers on the membrane surface that are used to identify subtypes of B cells (lymphocyte differentiation). The most common surface markers of B lymphocytes are CD19, CD20 and CD21.
CD19 as a 95-kDa member of the immunoglobulin super-family expressed exclusively on B lymphocytes is classified as a type I transmembrane protein, with a single transmembrane domain with a cytoplasmic C-terminus, and extracellular N-terminus. CD19 is a critical co-receptor for B cell antigen receptor (BCR) signal transduction. The surface density of CD19 is highly regulated throughout B cell development and maturation, until the loss of expression during terminal plasma cell differentiation. CD19 expression in mature B cells are 3-fold higher than that found in immature B cells, with slightly higher expression in B1 cells than in B2 (conventional B) cells.
B cells (CD19 lymphocytes) are a subgroup of lymphocytes and can be measured quantitatively in the blood as part of leukocyte typing (determination of immune status). The B cells fulfil their function within the framework of the so-called humoral immune system (production of antibodies).
The importance of the CD19 marker is that it allows analysis of B lymphocytes, which are responsible for humoral immune responses.
CD19 is one of the most reliable surface biomarkers for B lymphocytes they have an important role in the normal expansion and function of the peripheral B-cell pool. It contributes to maintaining the balance between humoral, antigen-induced response and tolerance induction, as even small modulations in CD19 expression can impact B cell signalling thresholds and dramatically affect the sensitivity and specificity of B cell mediated immunity.
Possible indications for determination of CD19 lymphocyte levels include:
Wang, K., Wei, G. & Liu, D. CD19: a biomarker for B cell development, lymphoma diagnosis and therapy. Exp Hematol Oncol 1, 36 (2012).
Xinchen Li, Ying Ding, Mengting Zi, Li Sun, Wenjie Zhang, Shun Chen, Yuekang Xu, CD19, from bench to bedside, Immunology Letters, Volume 183, 2017.