BLOOD!
Red Blood Cells vs. White Blood Cells
Red Blood Cells
Red blood cells are actually called Erythocytes (erytho means "red," cytes" means cell), and are often abbreviated "RBC." The main function of an RBC is to transport cells. RBCs have short lives; they only live up to four months.
White Blood Cells
White blood cells are called Leukocytes (white cell), and are abbreviated "WBC." WBCs main function is to fight disease and foreign invaders (antigens).