kidney
Your kidneys are organs that filter your blood to make urine. You have two of them, on either side of your abdominal cavity, toward your back.

liver
Your liver is a large organ that sits on the right side of your body, just below your rib cage. It does several important things, including processing and storing nutrients from food, and breaking down chemicals, such as alcohol.

immune system
The immune system is made up of the parts of the body that are devoted to fighting infection. The body is constantly being threatened by infections from things like bacteria, viruses and parasites. The immune system fights these infections in different ways. At the microscopic level, the immune system uses antibodies and white blood cells, which travel in the blood and target infectious agents, such as bacteria. These microscopic parts of the immune system either kill the infectious agent directly, or take it to other parts of the body, like the spleen, where it can be dealt with. The lymph nodes are another important part of the immune system. Within them, white blood cells filter through the foreign material that has entered the blood, to see if there are any infections. When you have a swollen gland during a cold, this is actually a lymph node that is reacting to the infection. Unfortunately, it is possible for the immune system to become confused and to use its destructive powers to target healthy parts of the body. Diseases that result from this type of situation are called autoimmune diseases.

viruses
Viruses are microbes (tiny organisms) that need the cells of humans or other animals to exist. They use the machinery of cells to reproduce. Then they spread to other cells in the body.

Epstein-Barr virus
Epstein-Barr virus (or EBV for short) is a common cause of infections in children and adolescents. It's the virus that causes mononucleosis or "mono." This infection can give you a sore throat, fever and swollen glands in your neck. If you've had an EBV infection, you may be more likely to get other conditions later in life, such as certain types of lymphoma (including Hodgkin's disease) and multiple sclerosis.

ataxia telangiectasia
Ataxia telangiectasia is a rare disease that affects children. It causes problems with walking, balance and speech. Children with the disease also have a weakened immune system, so they're more likely to get infections or certain types of cancer. Ataxia means clumsiness or poor coordination. Telangiectasias are tiny, red "spider" veins. People with the disease get these veins in the corners of their eyes or on the surface of their ears or cheeks.

Common variable immunodeficiency
Common variable immunodeficiency is a rare disease that affects children. Children with the disease don't make enough antibodies. The body needs antibodies to fight infections, so children with the disease are more likely to getting infections in their ears, lungs or sinuses. They also have a higher risk of getting certain types of cancer. The symptoms of the disease are very different for each child affected, which is why it's called "variable." Some children don't get any symptoms until they reach adulthood.

Severe combined immunodeficiency
Severe combined immunodeficiency is a rare disease that affects children's immune system. Children with the disease get life-threatening infections, like blood poisoning, lung infections or meningitis. It is sometimes called "bubble boy" disease, in memory of David Vetter, a boy with severe combined immunodeficiency who lived for 12 years in a plastic, germ-free bubble. The most common form of the disease only affects boys.

Wiskott-Aldrich syndrome
Wiskott-Aldrich syndrome is a rare disease that affects children. About 9 in 10 children with the disease are boys. Children with the disease have a weakened immune system. So they're more likely to get infections in their lungs or sinuses. And they often get eczema and bleed easily. The disease also makes it more likely that children will get certain types of cancer.

X-linked lymphoproliferative syndrome
X-linked lymphoproliferative syndrome is a rare disease that affects only boys. It's triggered by a virus called Epstein-Barr virus. Boys with X-linked lymphoproliferative syndrome make too many lymphocytes, cells in your blood that fight infections. And they don't make enough antibodies. The body needs antibodies to fight infections, so children with the disease are more likely to get infections. They are also likely to get a type of cancer called lymphoma.