histiocytoses

Your immune system is a complex network of cells, tissues, and organs. Together they help the body fight infections and other diseases.

When germs such as bacteria or viruses invade your body, they attack and multiply. This is called an infection. The infection causes the disease that makes you sick. Your immune system protects you from the disease by fighting off the germs.

The immune system has many different parts, including:

• Your skin, which can help prevent germs from getting into the body
• Mucous membranes, which are the moist, inner linings of some organs and body cavities. They make mucus and other substances which can trap and fight germs.
• White blood cells, which fight germs
• Organs and tissues of the lymph system, such as the thymus, spleen, tonsils, lymph nodes, lymph vessels, and bone marrow. They produce, store, and carry white blood cells.

Your immune system defends your body against substances it sees as harmful or foreign. These substances are called antigens. They may be germs such as bacteria and viruses. They might be chemicals or toxins. They could also be cells that are damaged from things like cancer or sunburn.

When your immune system recognizes an antigen, it attacks it. This is called an immune response. Part of this response is to make antibodies. Antibodies are proteins that work to attack, weaken, and destroy antigens. Your body also makes other cells to fight the antigen.

Afterwards, your immune system remembers the antigen. If it sees the antigen again, it can recognize it. It will quickly send out the right antibodies, so in most cases, you don't get sick. This protection against a certain disease is called immunity.

There are three different types of immunity:

• Innate immunity is the protection that you are born with. It is your body's first line of defense. It includes barriers such as the skin and mucous membranes. They keep harmful substances from entering the body. It also includes some cells and chemicals which can attack foreign substances.
• Active immunity, also called adaptive immunity, develops when you are infected with or vaccinated against a foreign substance. Active immunity is usually long-lasting. For many diseases, it can last your entire life.
• Passive immunity happens when you receive antibodies to a disease instead of making them through your own immune system. For example, newborn babies have antibodies from their mothers. People can also get passive immunity through blood products that contain antibodies. This kind of immunity gives you protection right away. But it only lasts a few weeks or months.

Sometimes a person may have an immune response even though there is no real threat. This can lead to problems such as allergies, asthma, and autoimmune diseases. If you have an autoimmune disease, your immune system attacks healthy cells in your body by mistake.

Other immune system problems happen when your immune system does not work correctly. These problems include immunodeficiency diseases. If you have an immunodeficiency disease, you get sick more often. Your infections may last longer and can be more serious and harder to treat. They are often genetic disorders.

There are other diseases that can affect your immune system. For example, HIV is a virus that harms your immune system by destroying your white blood cells. If HIV is not treated, it can lead to AIDS (acquired immunodeficiency syndrome). People with AIDS have badly damaged immune systems. They get an increasing number of severe illnesses.

Interstitial lung disease is the name for a large group of diseases that inflame or scar the lungs. The inflammation and scarring make it hard to get enough oxygen. The scarring is called pulmonary fibrosis.

Breathing in dust or other particles in the air is responsible for some types of interstitial lung diseases. Specific types include:

• Black lung disease among coal miners, from inhaling coal dust
• Farmer's lung, from inhaling farm dust
• Asbestosis, from inhaling asbestos fibers
• Siderosis, from inhaling iron from mines or welding fumes
• Silicosis, from inhaling silica dust

Other causes include autoimmune diseases or occupational exposures to molds, gases, or fumes. Some types of interstitial lung disease have no known cause.

Treatment depends on the type of exposure and the stage of the disease. It may involve medicines, oxygen therapy, or a lung transplant in severe cases.

The lymphatic system is a network of tissues and organs. It is made up of:

• Lymph - a fluid that contains white blood cells that defend against germs
• Lymph vessels - vessels that carry lymph throughout your body. They are different from blood vessels.
• Lymph nodes - glands found throughout the lymph vessels. Along with your spleen, these nodes are where white blood cells fight infection.

Your bone marrow and thymus produce the cells in lymph. They are part of the system, too.

The lymphatic system clears away infection and keeps your body fluids in balance. If it's not working properly, fluid builds in your tissues and causes swelling, called lymphedema. Other lymphatic system problems can include infections, blockage, and cancer.

Pigmentation means coloring. Skin pigmentation disorders affect the color of your skin. Your skin gets its color from a pigment called melanin. Special cells in the skin make melanin. When these cells become damaged or unhealthy, it affects melanin production. Some pigmentation disorders affect just patches of skin. Others affect your entire body.

If your body makes too much melanin, your skin gets darker. Pregnancy, Addison's disease, and sun exposure all can make your skin darker. If your body makes too little melanin, your skin gets lighter. Vitiligo is a condition that causes patches of light skin. Albinism is a genetic condition affecting a person's skin. A person with albinism may have no color, lighter than normal skin color, or patchy missing skin color. Infections, blisters and burns can also cause lighter skin.