Reactive blood changes are a heterogeneous group of pathological conditions that can occur in response to various conditions: viral, bacterial infections, helminth toxins, allergic agents. These reactions often require differential diagnosis with malignant neoplasms (MN) of the blood and hematopoietic organs (leukemias, lymphomas). It is important to note that only with a comprehensive examination of the patient with the involvement of a pediatric oncologist and hematologist, it is possible to exclude MN and carry out etiotropic treatment that contributes to the normalization of hemogram parameters. Article represents current data on the etiopathogenesis of leukemoid reactions, reactive thrombocytosis, secondary erythrocytosis and a clinical case observation of the development of a pseudoblast leukemoid reaction and reactive thrombocytosis against the background of an infectious process in a patient with Peutz-Jeghers syndrome, which will be of use to a wide range of pediatric practitioners.