The CD19 antigen (aka B-lymphocyte antigen CD19 or Cluster of Differentiation 19) plays an important role in clinical oncology. It’s a protein found on the surface of B-cells, a type of white blood cell.

Since CD19 is a hallmark of B-cells, the protein has been used to diagnose cancers that arise from this type of cell - notably B-cell lymphomas. CD19 has also been implicated in autoimmune diseases and may be a useful treatment target. CD19 is also expressed in a subset of acute myelogenous leukemias (=a cancer of the blood and bone marrow) indicating the close relationship between the lymphoid and myeloid lineages.

CD19 is present on B cells from earliest recognizable B-lineage cells during development to B-cell blasts but is lost on maturation to plasma cells. It primarily acts as a B cell co-receptor in conjunction with CD21 and CD81.

Quick notes:

- CD19 is one of the most reliable surface biomarkers for B lymphocytes. Its expression in mature B cell are 3-fold higher than that in immature B cells

- CD19 count may be useful to screen for B cell lymphoproliferative disorders (=conditions where lymphocytes are produced in excessive quantities.)

- CD19 is a biomarker for B cell development, lymphoma diagnosis and therapy. Lymphoma is a group of blood cancers that develop from lymphocytes

- CD19 plays a large role in regulating B-cell growth.

- CD19 is a biomarker for normal and neoplastic B cells, as well as follicular dendritic cells. 

- CD19 is a cell surface protein member of the large immunoglobulin family that complexes with CD21, CD81, and CD225 in the membrane of mature B-cells.

What are B-cells?

A B-cell is a type of white blood cell and, specifically, a type of lymphocyte.

Many B cells mature into what are called plasma cells that produce antibodies (proteins) necessary to fight off infections while other B cells mature into memory B cells.

B-cells fight bacteria and viruses by making Y-shaped proteins called antibodies, which are specific to each pathogen and are able to lock onto the surface of an invading cell and mark it for destruction by other immune cells. B-lymphocytes and cancer have what may be described as a love-hate relationship. For example, B-cells sometimes inhibit tumor development by producing antibodies that may attack cancer cells or oncogenic viruses, such as human papillomavirus (HPV), which is responsible for most cervical, anal, penile and other reproductive cancers. Other times, regulatory B-cells may release immune-suppressive cytokines that stifle an anti-tumor response. Also, B-cells are far more likely than T-cells to mutate into a liquid cancer such as chronic lymphocytic leukemia (CLL) or B-cell lymphoma.

CD19 is expressed by normal and neoplastic (=cancerous) B cells but is not expressed by T cells, monocytes, or granulocytes. CD19 protein appears early during B-cell maturation and is found during all stages of B-cell maturation, including plasma cells. CD19 is useful as an additional marker of B cell lineage in leukemias and lymphomas. Expression of CD19 may be seen in some acute myeloid leukemias.

Physiological function of CD19:

- CD19 is critically involved in establishing intrinsic B cell signaling thresholds through modulating both B cell receptor (=BCR)-dependent and independent signaling.

- It plays roles in the antigen-independent development as well as the immunoglobulin-induced activation of B cells. CD19 is thus critical for the body to mount an optimal immune response.

- CD19 works in complex with the BCR and other surface molecules to allow both direct and indirect recruitment and binding of various down-stream protein kinases.

References:

- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3520838/