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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.

You have two kidneys, each about the size of your fist. They are near the middle of your back, just below the rib cage. Inside each kidney there are about a million tiny structures called nephrons. They filter your blood. They remove wastes and extra water, which become urine. The urine flows through tubes called ureters. It goes to your bladder, which stores the urine until you go to the bathroom.

Most kidney diseases attack the nephrons. This damage may leave kidneys unable to remove wastes. Causes can include genetic problems, injuries, or medicines. You have a higher risk of kidney disease if you have diabetes, high blood pressure, or a close family member with kidney disease. Chronic kidney disease damages the nephrons slowly over several years. Other kidney problems include:

• Cancer
• Cysts
• Stones
• Infections

Your doctor can do blood and urine tests to check if you have kidney disease. If your kidneys fail, you will need dialysis or a kidney transplant.

NIH: National Institute of Diabetes and Digestive and Kidney Diseases

Vasculitis is an inflammation of the blood vessels. It happens when the body's immune system attacks the blood vessel by mistake. It can happen because of an infection, a medicine, or another disease. The cause is often unknown.

Vasculitis can affect arteries, veins and capillaries. Arteries are vessels that carry blood from the heart to the body's organs. Veins are the vessels that carry blood back to the heart. Capillaries are tiny blood vessels that connect the small arteries and veins.

When a blood vessel becomes inflamed, it can:

• Narrow, making it more difficult for blood to get through.
• Close off completely so that blood can't get through.
• Stretch and weaken so much that it bulges. The bulge is called an aneurysm. If it bursts, it can cause dangerous bleeding inside the body.

Symptoms of vasculitis can vary, but usually include fever, swelling and a general sense of feeling ill. The main goal of treatment is to stop the inflammation. Steroids and other medicines to stop inflammation are often helpful.

NIH: National Heart, Lung, and Blood Institute

Your immune system protects you from disease and infection by attacking germs that get into your body, such as viruses and bacteria. Your immune system can tell that the germs aren't part of you, so it destroys them. If you have an autoimmune disease, your immune system attacks the healthy cells of your organs and tissues by mistake.

There are more than 80 types of autoimmune diseases. They can affect almost any part of your body. For example, alopecia areata is an autoimmune disease of the skin that causes hair loss. Autoimmune hepatitis affects the liver. In type 1 diabetes, the immune system attacks the pancreas. And in rheumatoid arthritis, the immune system can attack many parts of the body, including the joints, lungs, and eyes.

No one is sure why autoimmune diseases happen. But you can't catch them from other people.

Autoimmune diseases do tend to run in families, which means that certain genes may make some people more likely to develop a problem. Viruses, certain chemicals, and other things in the environment may trigger an autoimmune disease if you already have the genes for it.

Millions of Americans of all ages have autoimmune diseases. Women develop many types of autoimmune diseases much more often than men. And if you have one autoimmune disease, you are more likely to get another.

The symptoms of an autoimmune disease depend on the part of your body that's affected. Many types of autoimmune diseases cause redness, swelling, heat, and pain, which are the signs and symptoms of inflammation. But other illnesses can cause the same symptoms.

The symptoms of autoimmune diseases can come and go. During a flare-up, your symptoms may get severe for a while. Later on, you may have a remission, which means that your symptoms get better or disappear for a period of time.

Doctors often have a hard time diagnosing autoimmune diseases. There's usually not a specific test to show whether you have a certain autoimmune disease. And the symptoms can be confusing. That's because many autoimmune diseases have similar symptoms. And some symptoms, such as muscle aches, are common in many other illnesses. So it can take a long time and some visits to different types of doctors to get a diagnosis.

To help your doctor find out if an autoimmune disease is causing your symptoms,:

• Learn about the health conditions in your family history. What health problems did your grandparents, aunts, uncles, and cousins have? Write down what you learn and share it with your doctor.
• Keep track of your symptoms, including how long they last and what makes them better or worse. Share your notes with your doctor.
• See a specialist who deals with the symptoms that bother you most. For example, if you have rash, see a dermatologist (skin doctor).

The treatment depends on the disease. In most cases, the goal of treatment is to suppress (slow down) your immune system, and ease swelling, redness, and pain from inflammation. Your doctor may give you corticosteroids or other medicines to help you feel better. For some diseases, you may need treatment for the rest of your life.

Your skin is your body's largest organ. It covers the entire outside of your body. There are many ways that your skin protects your body and helps keep you healthy. For example, it:

• Holds body fluids in, which helps prevent you from getting dehydrated
• Keeps out harmful germs, which helps prevent infections
• Helps you feel things like heat, cold, and pain
• Helps control your body temperature
• Makes vitamin D when the sun shines on it
• Shields your body against heat and light

There are many different problems and conditions which can affect your skin. Some of them can cause uncomfortable symptoms, such as itching, burning, redness, and rashes. They might also affect your appearance. Some of the more common skin conditions include:

• Acne, which causes pimples when hair follicles under your skin get clogged up
• Burns
• Cuts and scrapes
• Dandruff, flaking of the skin on your scalp (the top of your head)
• Eczema (atopic dermatitis), which causes inflammation, redness, and irritation of the skin
• Hives, which are red and sometimes itchy bumps on your skin
• Insect bites
• Psoriasis, which causes itchy, scaly red patches
• Skin cancer
• Skin infections

Since your skin protects your body in many ways, it's important to try to keep your skin healthy. For example, you can:

• Wear the right protective equipment, like gloves, long sleeves, knee and elbow pads, or helmets to protect against cuts, bumps and scrapes.
• If you do get a cut or scrape, clean it right away with soap and warm water. Put on a bandage to protect it while it heals.
• When you are spending time outdoors, wear long sleeves and pants and use insect repellant to prevent insect bites.
• Prevent sunburn by covering up and using sunscreen when outdoors.
• Wash your hands often with soap and water.
• When you take a shower or bath, use warm (not hot) water. Use mild cleansers and wash gently (don't scrub).
• Use moisturizers, like lotions, creams, or ointments, to prevent dry skin.

NIH: National Institute of Arthritis and Musculoskeletal and Skin Diseases