Bordetella pertussis infection in a child showing three reactive lymphocytes that are small with deeply cleaved nuclei and scant cytoplasm, in contrast with the monocyte in the lower right-hand corner. Reprinted with permission from I Pereira, TI George, DA Arber. Atlas of Peripheral Blood. The Primary Diagnostic Tool. Wolters Kluwer, Lippincott Williams and Wilkins, Inc., Philadelphia, PA (2012), page 130, Figure 13.5.