Source: Wan-Jen Hong, MD
Writing and editing by Rosanne Spector
Illustration by Bryan Christie Design
As you read this sentence, more than 20 trillion blood cells are carrying out widely varying missions to keep you alive. Most of the cells in this horde are red blood cells, stoking your body with oxygen. Other cells stanch blood loss and fight incursions of infectious agents and cancer cells. An average adult’s body holds just over a gallon of blood; 45 percent of that blood consists of cells. All told, those blood cells make up more than a quarter of the cells in your body. Each cell plays many roles — in fact, scientists have yet to figure them all out. But if you want to understand how a body works, blood cells are a very good place to start.
Born in bone: Blood cells are formed in the soft insides of bones, called the marrow. Once the cells are mature, they leave through tubes called venous sinuses, which empty into blood vessels. Cells that will become T lymphocytes complete their development in the thymus, a gland resting atop the heart.
Hematopoietic stem cells are the blood cells’ basic model. These simple cells can turn into all of the other types of blood cells.
Eosinophils combat multicellular parasites, such as hookworms. They also set off allergic reactions like asthma and hay fever.
Red blood cells carry the oxygen you inhale to all of the tissues in your body, where other cells depend on oxygen as fuel. They’re by far the most common blood cell. When they bind with oxygen, they turn bright red.
Basophils release chemicals that cause allergic symptoms. Among these are histamine, which spurs blood flow. Like eosinophils, they help fight parasitic infections. They’re the rarest of the white blood cells.
Platelets are fragments of cells that gather at the site of an injury and stick to the wounded blood vessel’s lining, creating the foundation for a clot. They also attract other immune cells to sites of injury or infection.
Natural killer cells act in a similar way to cytotoxic T cells, killing virally infected cells and tumor cells, but they spring into action faster. Instead of the three days to a week it takes for T cells to act, they go to work within minutes or hours.
Neutrophils are first on the scene to clean up after bacterial infection or injury. They’re the most common type of white blood cell, making up 50 to 70 percent of the total. These cells are a major component of pus, accounting for its yellowish color.
B lymphocytes churn out antibodies, large proteins that can identify and destroy bacteria, viruses and the toxins they make.
T-helper cells, also known as CD-4 cells, signal other immune cells to act.
Regulatory T cells, also known as CD-8 cells and suppressor T-cells, keep the immune system in balance, preventing it from attacking the body’s own parts.
Cytotoxic T cells release chemicals that kill virally infected cells
Memory T cells retain an impression of infectious agents they’ve encountered. If they meet that agent again, they quickly multiply and gear up to respond.
Macrophages (left) and dendritic cells surround and destroy bacteria and viruses, essentially eating them up. They also attach bits of the destroyed invaders on their surfaces, which alert the immune system to be on the lookout.
How clotting happens: Platelets gathering at the site of an injury sprout elongated growths on their surface, line the injured blood vessel and clump together to form a platform for clotting. Meanwhile a chain of protein interactions creates fibrin proteins (blue), which mesh with the platelets to stabilize the clot.