This document provides an overview of white blood cells (WBCs), also known as leukocytes. It discusses the discovery of WBCs, the types of WBCs including granulocytes and agranulocytes, the formation and development of WBCs through hematopoiesis and leukopoiesis, the morphology and functions of different WBC types such as neutrophils, eosinophils, basophils, lymphocytes and monocytes. It also addresses the regulation of leukopoiesis, metabolism of WBCs, normal WBC counts and percentages, and the kinetics and lifespan of neutrophils.