The nuclei of hairy cells are small, round, or oval, with irregular contours and have inconspicuous nucleoli. Cytoplasm may be lightly eosinophilic (as seen here). Presence of such cells as well as pools of blood in the red pulp are important diagnostic features. Electron microscopy shows cytoplasmic villi which appear hair-like on light microscopy, giving the cell its name. Hairy cells have a mature B-cell phenotype and express pan B-cell antigens.