Lymphocytosis refers to an increase of peripheral blood lymphocytes, which for adults is defined as an absolute lymphocyte count (ALC) > 4.0 x 109 /L.

Lymphocytes generally constitute 8-33% of white blood cell count (WBC) in peripheral blood. The normal number and distribution of lymphocyte subsets vary with age.

The major lymphocyte subsets are:

  1. T-cells: thymus-derived, important for Antigen recognition and binding 60 to 80% of the ALC
  2. B–cells: bone marrow-derived, produces immunoglobulins (“antibodies”), 10 to 20% of the ALC
  3. Natural Killer (NK)-cells: cytotoxic cells (large granular lymphocytes – LGL) that are part of the Innate resistance/ first line defence, 5 to 10% of the ALC

Lymphocyte differentiation and maturation are life-long processes that take place in the bone marrow and secondary lymphoid organs (lymph nodes, spleen, and thymus).