Red blood cells and white blood cells are, in essence, completely different. While both are necessary for the body's proper functioning, they each have singular roles. Red blood cells carry oxygen, while white cells do not, for example. Red blood cells in humans do not have nuclei, while white cells do.

Red blood cells, also called erythrocytes, are responsible for the characteristic color of our blood. They are responsible for picking up carbon dioxide from our blood and for transporting oxygen. The essential component of red blood cells is hemoglobin, which can hold oxygen so the cells can then transport around the body. This process is what gives the body energy, which explains why people who suffer from anemia — low count red blood cells — often feel tired and sleepy. A high count of red blood cells is rare, but it can happen. Causes include kidney disease, dehydration, anabolic steroid use, and pulmonary fibrosis. People suffering from a high count of red blood cells usually have impaired circulation, and are at a high risk for heart disease.

While blood cells or leukocytes, on the other hand, are primarily responsible for fighting foreign organisms that enter the body. This includes everything from bacterial and parasitic infections to allergic response. T-cells, a form of white blood cells, are the ones that stop functioning properly in the presence of an HIV infection. An overproduction of white blood cells can lead to leukemia. On the other hand, certain medications, such as Clozapine®, used in psychiatry, can reduce the number of white cells significantly.

There are approximately 5 million red blood cells in every cubic millimeter of blood; there are only 3,000 - 7,000 white blood cells in the same amount of blood. Red blood cells have an average lifespan of 120 days, while white cells live a maximum of four days.

Red blood cells have a circular shape that resembles a shallow bowl, but they can change shape without breaking to squeeze through smaller spaces if necessary. White blood cells have different shapes, depending on their function. While they can multiply easily, they don't change shape.