White blood cells (WBCs, leukocytes) form the first line of defense of the body against invading microorganisms. Neutrophils and monocytes respond by phagocytosis; lymphocytes and plasma cells primarily produce antibodies. In addition to a nonspecific response to bacterial or viral infection, there are alterations in the normal leukocyte blood picture that may provide diagnostic clues to specific diseases, both benign and malignant. Nonneoplastic leukocyte alterations may be quantitative, qualitative, or both; qualitatively, leukocytes may demonstrate an increased degree of immaturity, morphologic alteration in cellular structure, or the increased production of less common types of WBCs.