White blood cell count disorders form a spectrum from leukopenia (decreased WBC count) to leukocytosis (increased WBC count), each subclassified by which cell lineage—neutrophils, lymphocytes, eosinophils, basophils, or monocytes—is affected, reflecting distinct underlying pathophysiologic processes such as acute inflammation, allergy/helminthic infection, viral infection, chronic inflammation, or myeloproliferative disease. Neutrophilic leukocytosis in particular is defined by an absolute neutrophil count above normal limits and is characterized by a "left shift" (release of immature band cells from the bone marrow), with extreme presentations distinguished as leukemoid reaction (a benign, functional neutrophilia mimicking leukemia, marked by elevated leukocyte alkaline phosphatase) versus leukoerythroblastic reaction (release of immature white and red cell precursors, including nucleated RBCs and teardrop cells, due to bone marrow space-occupying infiltration such as myelofibrosis or metastatic cancer).
White blood cell count disorders form a spectrum from leukopenia (decreased WBC count) to leukocytosis (increased WBC count), each subclassified by which cell lineage—neutrophils, lymphocytes, eosino…