Lymphocyte count

The absolute lymphocyte count (ALC) is the total number of circulating lymphocytes — comprising T cells, B cells, and NK cells — derived from a complete blood count differential. In adults, the normal range is approximately 1.0–4.0 × 10⁹/L. Lymphopenia (ALC <1.0 × 10⁹/L) is a recognized marker of immune deficiency and occurs in HIV infection, autoimmune diseases, lymphoma, following chemotherapy or radiotherapy, and in severe malnutrition. With aging, the absolute count tends to decline and the composition shifts — naïve T cells decrease while memory and effector cells accumulate — a constellation termed immunosenescence. Population studies consistently link lower lymphocyte count with higher all-cause mortality, greater frailty, and increased infection vulnerability, and ALC is a component of several validated biological aging indices. Importantly, lymphopenia was a strong predictor of severe COVID-19, underscoring its broader role as a marker of immune reserve.