Blood is a useful but complicated compound.

There’s a paradox about blood: while most of us are more interested in the lungs or heart, both of these organs only perform their functions to serve the blood. That is, the lungs are there to introduce oxygen into the blood, while the heart moves blood around the body.

Blood is the Swiss Army knife of our bodies: it’s an everyday tool with many functions.

Our 30 trillion red blood cells, which give blood its distinctively rich color, deliver oxygen to the body’s organs and tissues. These cells also conduct waste disposal, carrying the used oxygen – now converted into carbon dioxide – back to the lungs for removal. In a single day, these cells travel a combined total of 12,000 miles. 

Blood contains other cells, too. Platelets, for example, will move to a site of bleeding to help clot the blood and stop the flow. When you injure yourself and see the bleeding slow down, you’re witnessing a dazzling biological dance involving millions of platelets and proteins.

There are also white blood cells – the weapons of our immune system. When our bodies detect a threat, whether from a virus, bacteria or toxin, white blood cells attach themselves to the intruder and digest it. 

Given all the valuable functions of blood, it’s no wonder that blood transfusions are a crucial procedure in medical care. We transfuse red blood cells into transplant patients to help their bodies accept new organs, and cancer patients with too few platelets are infused with millions more. Globally, someone receives a stranger’s blood every three seconds. 

But we can’t transfuse a patient with just any blood.

All blood has a type, which is determined by antigens, that is, molecules that stick to the surface of red blood cells. But antigens come in different types. All blood cells have H antigens; it’s the combination of A and B antigens that determines blood type. Type A blood has only A antigens, and Type B has only B. Type AB has both, while Type O has neither.

Type is then further classified according to the Rhesus factor, which identifies the blood as either “positive” or “negative” depending on its specific cocktail of antigens. Our bodies will only accept blood of the correct type. The body of a patient with A negative blood will reject a transfusion of B positive. In the most severe cases, the patient can even go into hemolytic shock and die.

Leeches, however, welcome all blood types.