The leukocytes consist of neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Neutrophils are granulocytes. Neutrophils have a multi-lobed nucleus. The lobes are separated by a thin strand. Because of the shape of the nucleus, neutrophils are also called "polymorphonuclear neutrophils", "poly's", "PMN", or "polymorph. Lymphocytes are agranulocytes. Lymphocytes can be categorized by three sizes: small, medium, and large. The small lymphocytes are the smallest leukocyte, being only slightly larger than an erythrocyte. Monocytes are agranulocytes. They are the largest leukocyte. Eosinophils are granulocytes. Eosinophils have prominent orange pink granules. The nucleus of eosinophils is usually bi-lobed.
Basophils are granulocytes. Basophils have large blue granules, which often obscure the nucleus. The granules are basophilic, therefore they are blue.