Lymphocyte

Most blood lymphocytes are small lymphocytes (≈ 6–9 µm, about the size of a red cell) with a round, densely staining nucleus occupying almost the whole cell and a thin rim of pale blue cytoplasm; large lymphocytes (≈ 10–14 µm) include activated cells and the large granular lymphocytes (mostly NK cells and some cytotoxic T cells). The main types — T cells (≈ 70–80% of blood lymphocytes), B cells (≈ 10–15%) and NK cells (≈ 5–15%) — look identical under the light microscope and are distinguished by surface markers using flow cytometry (CD3 for T cells, CD19/CD20 for B cells, CD56/CD16 for NK cells). Innate lymphoid cells (ILC1–3) are rarer tissue-resident relatives.

Only about 2% of the body's lymphocytes are in the blood at any one time; the rest are in lymphoid organs and tissues, and many recirculate continuously between blood and lymph. Naive lymphocytes are quiescent, long-lived cells; on meeting their antigen they enlarge into lymphoblasts, proliferate and differentiate into effector cells (plasma cells, helper and cytotoxic T cells) and long-lived memory cells. Children have higher lymphocyte counts than adults; in infants lymphocytes may outnumber neutrophils.