Illustration of red and white blood cells. Red blood cells or erythrocytes transport hemoglobin, an iron rich pigment that has high oxygen carrying capacity. In this way red blood cells distribute oxygen to the body. Red blood cells also carry the waste gas carbon dioxide back to the lungs. White blood cells form part of the immune system and migrate from circulating blood into tissues to defend the body against invasion by foreign organisms. In an active state, the cell surface emits pseudopodia allowing the cells to move toward an infectious center.