This document summarizes the three main types of blood cells: red blood cells, white blood cells, and platelets. Red blood cells constitute 40-45% of blood, are disc-shaped and red in color due to hemoglobin, lack a nucleus, and function to transport respiratory gases. White blood cells constitute 1% of blood, are colorless and nucleated, and function in immunity. Platelets are colorless, lack a nucleus, and function in blood clotting. The document compares characteristics of red and white blood cells and provides details on composition, shape, development, lifespan and function of each blood cell type.