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: