Outside your body, foreign invaders are waiting for opportunities to spread. Bacteria, viruses, parasites, and fungi can all thrive inside your body—often at your expense. Your immune system keeps them away from your healthy cells, protecting you from infections. It tries to keep those invaders out. When it can't, your immune system attacks the harmful pathogens that spread sickness. To do this, your body uses a wide variety of cells and a network of organs all communicating to keep threats away.
To take advantage of your healthy cells, germs need to find a way inside your body. There are only a few paths in. Sometimes they enter your skin from a cut or a bug bite. Sometimes they come from foods you eat or drink, and may cause food poisoning. Sometimes when you swim, pathogens get into your eyes, ears, nose, or mouth. Sometimes you breathe them into your lungs. And sometimes you touch germs and then rub your eyes, nose, or eat with your hands. You can catch germs in all these ways.
Your skin is like an elastic shield around your whole body. This is your first line of defense against germ invaders. But your skin shield can be broken, whether it's from a paper cut or a mosquito bite. Keep cuts clean, dry, and bandaged to help prevent skin infections.
Your body makes all kinds of liquids that flush germs away. Consider the tears in your eyes, the mucus in your nose, and the sweat on your skin. These all push germs out and away from your body. And they all secrete a special antibody—IgA—that guards against infection.
Lymph is a clear fluid that washes across your cells. Lymph fluid contains T-cells, which both direct your immune response, and also attack infections and cancer cells. Lymph travels through lymphatic vessels and blood vessels. Bean-shaped lymph nodes are clustered around your underarms, groin, and neck. Lymph nodes are where T-cells and other white blood cells concentrate, and where foreign cells like microbes, dead cells, and cancer cells are removed from the blood stream and destroyed.
An antigen can be anything that triggers an immune response. That includes viruses and bacteria, but also foreign tissue (as from an organ transplant). Your immune system can make mistakes, like mistaking a harmless plant spore as a pathogen or even some of your healthy tissue for an invader. When this happens, autoimmune diseases like type 1 diabetes or lupus may result.
Your innate immune system is your first defense against infection. It's made of the parts of your immune system that respond immediately to infections or within a few hours. The innate immune system includes your skin and other physical barriers. It also includes some cells that activate your immune response so that invading germs can be attacked and destroyed.
When your body successfully fights off an infection, you acquire immunity to that infection. And when you get a shot of a vaccine or antibody serum, you may acquire immunity. In acquired immunity, some of your immune cells remember the infection or vaccine so that you are prepared to stop the invader if it reappears. Immunity may last a matter of months, or it could last a lifetime, depending on the disease. And it may be weak or strong depending on the antigen, its concentration, and how it enters your body.
Your bones would be hollow if it wasn't for bone marrow, the jellylike tissue inside them. Bone marrow is where most blood cells come from. Blood cells start as stem cells inside your bone marrow before they mature and specialize. That includes your white blood cells, which are essential to immunity.
When your immune system senses an invader, it sends a phagocyte to gobble it up. A phagocyte is a white blood cell that devours an invading germ. Phagocytes include both the largest white blood cells (monocytes) and the most common white blood cells (neutrophils).
Natural Killer Cells (NK cells) are a type of lymphocyte that destroys antigens and may kill other cells that it recognizes as foreign. But unlike other lymphocytes, NK cells don't need to be told what to attack. This helps them fight a wide variety of foreign cells.
When you have allergies, basophils and mast cells are responsible for your symptoms. These are also effective against parasites. Parasites are usually much bigger than bacteria and viruses. So these more generalized immune cells are more effective against them.
Lymphocytes are white blood cells that pose a deadly threat to infections. Lymphocytes are crucial to your immune system, and make up about 20% to 40% of your white blood cell supply. They can bind to billions of foreign substances, and can clone themselves to make more disease-fighting copies once they do. Some of these cells remember past infections and can quickly multiply if an infection returns. Some lymphocytes have been gene-engineered to fight cancer.
Lymphocytes can cause problems when your body has too few or too many of them. A normal count for adults is between 1,000 and 4,800 for every microliter of blood. Children normally have between 3,000 and 9,500 lymphocytes per microliter.
- Lymphocytopenia: This disorder is caused by a lack of lymphocytes in your blood. It can be mild or severe, and fever is the most common symptom.
- Lymphocytosis: Your blood may have high numbers of lymphocytes shortly after you fight off an infection, and that is harmless. However, lymphocytosis can also be caused by many conditions, and some of these are deadly. They include:
Antibodies do a lot of important tasks. Which task? It depends on their specialty. They may kill bacteria or help other cells digest microbes. Some antibodies mark invading cells for other immune cells to attack. These are sometimes called Ig (immunoglobulin). The three major antibody types are IgA, IgM and IgG.
T cells are one main type of lymphocyte. T cells can either coordinate the destruction of cells or attack them directly. There are two main types of T cells.
- Helper T cells: These immune cells live up to their name. They can order phagocytes to devour microbes. They can drive the production of antibodies. And they can trigger more helpers.
- Killer T cells (CTLs): These blood cells find infected cells and kill them, including some cancerous cells.
Your thymus sits high in your chest behind your breastplate. T cells thrive in this lymphatic organ, where they develop and multiply. This is where T cells learn the difference between your healthy cells and foreign cells, and where they learn to tell antigens apart.
Your spleen is an organ that sits in the upper left of your abdomen. It filters foreign cells and has special compartments for immune cells to gather and work. Like the lymph nodes, the spleen is a battleground between immune cells and antigens.
Your tonsils and appendix are some of the many areas in your body where lymphoid tissue clusters together. Other areas include your airways and digestive tract lining. These are gateways in your body, so these organs are like gatekeepers patrolling for infectious agents.
Your white blood cells spike during and right after an infection, but most of them don't last long inside your body. After some time, most self-destruct. But a few remain for much longer. These remaining cells are called memory cells. They give your body the blueprint for an immune response if the infection ever returns.
The complement system is a network of about 30 proteins. They complement the work of your antibodies by responding to and destroying bacteria and antigens that are bound to antibodies. They do this through a complex series of interactions known as the complement cascade. This cascade forms a tight cylinder of proteins that pierces a cell wall, causing it to swell and burst. The complement system also contributes to the redness, pain, and swelling of inflammation.
Cytokine proteins are chemical messengers. They perform lots of functions. Some turn other immune cells on and off. Other cytokines gather at the site of an infection or injury, sending chemical signals to attract other immune cells. Cytokines often contribute to inflammation.
Your immune system is complicated, and relies on a lot of chemical interactions to go right. They don't always. Sometimes you react to a harmless foreign substance like ragweed pollen, which is an allergic reaction. Sometimes immune system mistakes result in immune disorders, which can be divided into two types. An overreaction of the immune system that damages your body is known as an autoimmune disorder. A weakened immune response is known as an immunodeficiency disorder.
- Autoimmune disorder: Autoimmune disorders can be caused by a mixture of genetics and the environment. These disorders include Crohn's disease, multiple sclerosis, and type 1 diabetes.
- Immunodeficiency disorders: These are usually caused by a chronic disorder like cancer or from use of a drug, and are sometimes inherited. These disorders weaken the body's immune system, making it less capable of fighting off bacteria, viruses, cancer cells, and other harmful cells. They can be caused by leukemia, HIV/AIDS, and other prolonged illnesses including diabetes.
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