Morphologically abnormal lymphocytes must be differentiated from normal lymphocytes. Atypical lymphocyte are either inflammatory reactive cells or malignant cells within the manifestation of lymphatic neoplasms of mature cells.
For a long time, there was no unified nomenclature for atypical lymphocytes. The term was used as well for inflammatory reactive lymphocytes as for generally conspicuous lymphocytes.
A German group of experts (Arbeitskreis Laboratorium der DGHO) proposed in 2011 a new unified nomenclature. In a first step, normal lymphocytes are differentiated from morphologically abnormal Lymphocytes. If cytologically possible, the abnormal cells should be differentiated in a second step into "atypical lymphocytes" of probably reactive nature and "atypical lymphocytes" of probably neoplastic nature.
Reactive atypical lymphocytes can vary widely in size, form and shape. They are larger than normal lymphocytes and have a large volume of light to dark basophilic cytoplasm that has a more intense color along the cell membrane. Vacuoles and azure granules may also be present. The nucleus is often kidney-shaped. The chromatin is finer than in small lymphocytes and often has one or more nucleoli. Atypical lymphocytes can not always be clearly identified and are often confused with leukemic blasts by those who are inexperienced. In contrast to blasts, the chromatin of atypical lymphocytes it not fine enough and their appearance is too polymorphic. Atypical lymphocytes can belong to the B-call line as well as to the T-cell line.
- Viral infections such as infectious mononucleosis, cytomegalovirus and primary HIV infection
All morphologically altered lymphocytes in the peripheral blood, that can not be clearly identified as large granular lymphocytes (LGL) or reactive atypical lymphocytes, are suspected to be mature cells of lymphatic neoplasms (= dissemination of lymphoma cells). The appearance of such cells can vary widely. They can not always be a assigned to specific diseases. Occasionally, possibly neoplastic atypical lymphocytes are found in an otherwise normal lymphatic population.
Appearance of mature neoplastic B- and T-cells in the peripheral blood: