When your doctor orders a "complete blood count with differential" (CBC with diff), the laboratory examines the different types of white blood cells circulating in your blood.

Lymphocytes are the commanders of your immune system's targeted response. They typically make up about one-third of your white blood cells (1.0–4.0 × 10⁹/L) There are two main types:

• B cells produce antibodies—proteins that recognize and neutralize specific germs

• T cells directly attack infected cells and coordinate the immune response

High lymphocytes (lymphocytosis) commonly occur with viral infections (like the flu or mononucleosis), but can also indicate certain blood cancers like chronic lymphocytic leukemia. Low lymphocytes (lymphopenia) can occur with HIV infection, autoimmune diseases, or after chemotherapy.