Leukocytes, or white blood cells, are part of the innate immune system that defends against infectious and foreign agents. In pediatrics, it is important to use age-specific laboratory values when interpreting results. Infections are the most common cause of leukocytosis or leukopenia in children. Symptoms suggestive of more serious etiologies include persistent fevers, weight loss, bruising, fatigue, and adenopathy. Neutropenia is of special importance in pediatrics due to associations of severe neutropenia with genetic syndromes and overlapping presentations with primary immunodeficiencies. Although the discovery of novel genetic mutations has aided the hematologist/oncologist and the immunologist in managing these conditions, the relationship between clinical phenotype and mutation is still not well known. [Pediatr Ann. 2020;49(1):e17-e26.].
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