A blood cell, also called a hematocyte, is a cell produced by hematopoiesis and is normally found in blood. In mammals, these fall into three general categories:1  Red blood cells – Erythrocytes. 2  White blood cells – Leukocytes. 3  Platelets – Thrombocytes.hite blood cells are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five diverse types of leukocytes exist, but they are all produced and derived from multipotent cells in the bone marrow known as a hematopoietic stem cells.