Absolute B Cells (CD19+) Test

CD19+ B cells are a type of white blood cell that play a central role in your immune system. These cells are responsible for producing antibodies, proteins that help identify and neutralize harmful invaders like viruses, bacteria, or cancer cells. CD19 is a protein found on the surface of nearly all B cells, which makes it a reliable marker for identifying them in blood tests.

An “absolute” CD19+ B cell count refers to the total number of these B cells in a given amount of blood, usually measured using a technique called flow cytometry. This test doesn’t just give a percentage; it tells you the exact number of B cells, which is important for understanding whether your immune system is functioning normally, or if it’s overactive, underactive, or responding to a disease.

In autoimmune conditions like systemic lupus erythematosus (SLE), the body mistakenly attacks its own tissues. CD19+ B cells can contribute to this process by producing autoantibodies (antibodies that target the self) or failing to regulate inflammation properly. In healthy individuals, a specialized group of B cells called regulatory B cells help keep the immune system in check, but in people with lupus, these cells may be present in normal numbers but function poorly.

On the other hand, CD19+ B cells can also become cancerous. B-cell lymphomas and leukemias are cancers that arise from B cells, and in these cases, measuring CD19+ B cell counts can help diagnose the disease and monitor treatment. Because CD19 is such a specific marker for B cells, it has become a powerful therapeutic target. Treatments like CAR-T cell therapy and monoclonal antibodies are designed to find and destroy CD19+ B cells, both healthy and cancerous ones. These therapies can lead to a near-total depletion of B cells in the body, which is often a goal in diseases like lymphoma or certain autoimmune conditions. However, in some patients, mutations in CD19 can allow cancer cells to escape treatment, which has led researchers to develop dual-targeted therapies that go after more than one protein on the B cell.