Phagocytosis denotes a process by which cells engulf and internalize sizable extracellular objects, including bacteria. Phagocytes, the cells that carry out phagocytosis, include neutrophils, monocytes, macrophages, and eosinophils. These specialized white blood cells play an important role in defending the body against infection. Neutrophils can reside in the periphery or in the bone marrow, and biologic differences like the Duffy antigen can influence laboratory values of absolute neutrophil counts while having no impact on corresponding health status. In addition, there are a number of disorders of phagocytic function that can result in human disease. It is critical to distinguish disease from normal variation in healthy patients. Disorders of phagocytic function usually impair the immune system, so most fall under the more general category of primary immunodeficiencies.