#00063600

A 3-month-old male infant, born prematurely at 26 weeks and overdosed by Propanolol for Supraventricular Tachycardia, was found to have leukocytosis. A. Peripheral blood smear, Wright and Giemsa stain, (Cellavision DM image, magnification x1000), showing increase of atypical/reactive lymphocytes, slightly increase of cytoplasm, condensed chromatin, a few cells have fine chromatin, mimicking “blast”. B. Side Scatter versus CD45 on density plot, showing no increased of cells at blast gate, but increase of CD45+ lymphocytes. C. CD5 versus CD19 density plot, showing a CD5+ and CD19+ population, composed of 21% of the total population. D. CD20 versus CD45 dot lot, showing the CD5+/CD19+ lymphocytes that are strongly positive for CD20 and CD45. E. Lambda versus kappa dot plot, showing CD5+/CD19+ lymphocytes that are polyclonal B cells. They are also positive for IgM and cytoplasmic CD79a (not shown). No immature markers including CD34, TdT, or CD117 are identified; no T cell or other myelomonocytic markers, or CD10 are present on these B lymphocytes. Overall, the findings are compatible with Innate-like, polyclonal CD5+/CD19+ B1a lymphocytosis. B lymphocytes are divided to B1 innate and B2 mature cells as developing and involving innate and adaptive immune responses, respectively. Innate B1 cells are further classified as B1a and B1b based on the expression of CD5. B1a is CD5 positive while B1b is negative.