White blood cell. There are five different types of mature leukocytes: neutrophil, eosinophil, basophil, lymphocyte, and monocyte. Leukocytes protect the body from disease. Through a special process, leukocytes move into a tissue outside the blood vessels to carry out their disease-fighting function. Leukocytes are classified as either granulocytes: white blood cells that have granules in their cytoplasm (neutrophils, eosinophils and basophils); those without granules are calledagranulocytes (lymphocytes and monocytes).