Macrophages are the body’s biggest eaters. They overwhelm and kill off bacteria and  then clear dead cells and tissue debris. If overactivated, though, macrophages can trigger inflammatory disease and cancer growth. "They can be heroes or villains," says Cedars-Sinai immunologist Helen Goodridge, PhD, who notes that macrophages are one of her favorite cells.

Neurons are the longest cells, with the lengthiest stretching nearly 5 feet down the spinal cord. Aided by motor proteins, these nerve cells transmit electrochemical messages that create movement, thought and sensations, including pain. They go the distance in age too: Brain neurons can last an entire human lifespan and, theoretically, even longer. Compare that to the lifespan of heavily worked gut epithelial cells (three to five days) or pancreatic cells (one year).

Skin is constantly shedding and being replenished. The human body sloughs off 200 million dead cells from the skin’s outer layer every hour (almost 5 billion a day), replacing the skin entirely every 40 to 56 days.

About 70% of the human body’s cells—25 trillion—are red blood cells, or erythrocytes, making them the most common. That vast supply is critical to life: Tiny erythrocytes continuously ferry oxygen from the lungs throughout the whole body. The hemoglobin protein is what paints blood its trademark red.

Found in embryonic and certain adult tissues, stem cells have not yet specialized. These prolific cells are at the heart of regenerative medicine, with their potential to be manipulated into immune, bone, skin, nerve or muscle cells. There is evidence that, in certain circumstances, just one hematopoietic stem cell in the bone marrow could regenerate and sustain an individual’s entire blood supply, including red blood cells and more than a dozen types of immune cells, Goodridge notes.