Any of various cells that in adult mammals are produced in bone marrow, have a nucleus but no hemoglobin, and function in the immune system by protecting against pathogens and aiding in tissue repair. White blood cells include neutrophils, eosinophils, basophils, lymphocytes, and monocytes, and they are found in blood, lymph, and certain tissues. Also called leukocyte, white cell, white corpuscle.