When you cut yourself, red blood flows out of your body. Have you ever wondered what makes blood red?

Your blood is made of four things: plasma, platelets, white cells and red cells. Plasma is a yellow-colored liquid that carries the cells and also contains many important chemicals. Platelets are cells that clump together and form clots to stop the bleeding when you get a cut. White cells fight the many kinds of germs that get into your body. Red cells carry oxygen, an invisible gas in the air you breathe, to every part of your body.

Just one drop of your blood has a huge number of red cells in it, around 5 to 6 million! Each of the round red cells contains a chemical material called hemoglobin. It's actually the hemoglobin that makes your blood look red. When hemoglobin mixes with oxygen from the air, a chemical reaction takes place. The cells carrying hemoglobin turn red. Because your blood has so many red cells in it, the whole mixture -- red cells, white cells, platelets and plasma -- looks red.

Under a microscope, a red blood cell looks like a tiny, round pillow with a dent in the center. The cells are flexible and move freely through the system of vessels that carries blood to all the cells of the body. Each red cell lives about 120 days; red cells are constantly being produced by the body to replace the ones that wear out and die.

The red blood cells of people who have an illness called sickle cell disease do not look round. The blood of people with this illness contains an unusual kind of hemoglobin. Their red blood cells have a curved shape -- like a sickle -- instead of a round one.

As the sickle-shaped red cells travel through the bloodstream, they sometimes get stuck, causing a temporary blockage in small blood vessels -- a sickle cell crisis. When that happens, the sickle cell patient feels pain, especially in the hands and feet, the back, the chest and the abdomen. Sickle cell disease also causes the red blood cells to wear out and die faster than usual, leading to a condition called anemia. Anemia makes a person feel weak and sick and makes him or her more likely to catch colds and other infections.

Experts believe that sickle cell disease began thousands of years ago, probably in Africa or the Middle East. It has been passed on from generation to generation since then. Doctors call diseases that pass down this way inherited disorders. A person can only get sickle cell disease if this disorder has been passed down in his or her family; you can't catch sickle cell disease from being around someone who has it.

Sickle cell disease is found in several parts of the world, including Africa, Mediterranean countries like Greece and Turkey, South and Central America, Caribbean countries and southern India. In the United States, the majority of sickle cell patients are black.

According to the Center for Sickle Cell Disease at Howard University in the District, some 50,000 black Americans have the disease. About 2 million have sickle cell trait -- the capacity to pass the illness on to their children.

Right now, there is no simple cure for sickle cell disease. However, two children with sickle cell disease recently have been helped by an operation called a bone marrow transplant.

As doctors continue to work to find a cure, patients have to learn to live with the painful crises and frequent colds and other infections that the disease brings. This isn't easy: Having sickle cell disease often means missing school and having to go to the hospital for treatments. Between crises, however, a child with sickle cell usually feels fine and can do everything that other kids do. If you know someone with this illness, you can help by being understanding, including your friend in your activities and keeping in touch when sickle cell disease confines your friend to home or the hospital.Tips for Parents

The Center for Sickle Cell Disease at Howard University is involved in research, education, screening and clinical treatment. Among the center's publications is a booklet: "A Child With Sickle Cell Disease . . . His Hobbies and Activities" by Dr. Sylvia Tetrault. She writes, "Helping your child to find ways to use his or her spare time can be very important." She recommends the following home activities for kids with sickle cell disease: gardening, building models, arts and crafts, painting, playing a musical instrument, chess or checkers and reading. In addition, she writes, "The assignment of household chores will give our child a sense of responsibility, belonging and achievement. These feelings may be reinforced by giving the child a reward, such as an allowance." For more information, contact the Center for Sickle Cell Disease, Howard University, 2121 Georgia Ave. NW, Washington, D.C. 20059. Phone: 636-7930.

Catherine O'Neill is a free-lance children's writer based in Baltimore.