Reactive, or atypical lymphocytes are lymphocytes that become large as a result of antigen stimulation. Typically they can be more than 30 µm in diameter with varying size and shape. The nucleus of a reactive lymphocyte can be round, elliptic, indented, cleft or folded. The cytoplasm is often abundant and can be basophilic. Vacuoles and/or azurophilic granules are also sometimes present. Most often the cytoplasm is gray, pale blue or deep blue in colour. Reactive lymphocytes are usually associated with viral illnesses, however, they can also be present as a result of drug reactions, immunisations, radiation, hormonal causes (such as stress and Addison's disease) as well as some auto-immune disorders (such as rheumatoid arthritis). Some common pathogen-related causes include:Streptococcus, agalactiae (Group B Streptococci), Epstein-Barr virus, Treponema pallidum (Syphilis), Hepatitis.