Lymphocytosis refers to an increase in circulating lymphocytes. These cells may be small mature lymphocytes, reactive lymphocytes or atypical lymphocytes and therefore it is important to assess the morphology of these cells histologically.

Lymphopaenia is a reduction in circulating lymphocytes. This may be reactive, reflect immunodeficiency or occur secondary to drugs.

Mature lymphocytes are small cells with high nuclear : cytoplasmic ratio (i.e. a very small amount of cytoplasm). These cells are normally seen in peripheral blood however if present in a large number then they may suggest chronic lymphocytic leukaemia.

Reactive lymphocytes are medium to large cells with mature chromatin abundant cytoplasm 

Plasmacytoid lymphocytes, or Türk cells, are an intermediate stage between a B lymphocyte and a plasma cell.

The presence of Türk cells in peripheral blood is usually reactive, however they may be present in patients with Waldenstrom's macroglobulinaemia.

The term atypical lymphocyte is used to refer to any abnormal lymphocyte that does not appear reactive. The presence of such cells in the peripheral blood suggests a neoplastic process, such as a lymphoid leukaemia or the leukaemic phase of lymphoma. These cells may be difficult to distinguish on morphological examination.