Atypical mononuclear leukocytes (also referred to as atypical lymphocytes) are lymphocytes with some morphological properties of monocytes. They have one nucleus, meaning they are younger cells which goal is to fight viruses.
The presence of atypical mononuclear cells is the most common diagnostic criterion for infectious mononucleosis, although it may occur in other viral diseases as well. Even a simple viral infection may cause an increase in atypical mononuclear cells. However, in that case they are less than 10%. Generally, the diagnosis of infectious mononucleosis is confirmed if the white blood count has more than 10% of atypical mononuclear leukocytes.