Description:

Nucleocytoplasmatic ratio is more than normal, vacuolization is visible in the cytoplasm, cytoplasmic pseudopodii can be visible and the nucleolar chromatin is lumpy.
Cells can constitute up to 5% of the lymphocytes in healthy people, up to 15% in an acute viral infection; and more in infectious mononucleosis and some other diseases with hyperplasia of lymphoreticular tissue.
More than 50% of the leukocytes in the peripheral blood in patients with mononucleosis caused by Epstein-Barr or cytomegalo virus are lymphocytes, and more than 20% of them are atypical.
Minimum morphological criteria is: presence of 50% mononuclear cells (lymphocytes and monocytes), at least 10 atypical lymphocytes by 100 leukocytes, and outstanding polymorphic lymphocytes.        
In most patients 20-60% atypic reactive lymphocytes can be found during fever.

Size:

12-15 µm (larger than a large lymphocyte).

Cytoplasm:

Abundant, light blue, often polygonal, with darker colored edges and often azurophilic granules.

Nucleus:

Eccentrically located, chromatin is tender, often with a visible nucleolus. The nucleus can be round, with coarser chromatin.

Nucleoli:

Often visible.