SafekipediaThe immune system is a network of biological systems that protects an organism from diseases. It finds and stops many harmful things, like viruses, bacteria, and parasites. It also fights cancer cells and foreign objects, such as wood splinters. This system helps keep us healthy by fighting off germs and other unwanted visitors.
Many species have two main parts of the immune system. The innate immune system gives a ready response to many different threats. The adaptive immune system learns and remembers each threat it meets before. Both use molecules and cells to do their work, teaming up to protect the body.
Nearly all organisms have some kind of immune system. Even bacteria have a simple way to guard against viral infections. More complex animals, including humans, have a advanced defense system. For example, adaptive immunity lets the body learn and remember past invaders, so it is ready if they return. This is why vaccination works well—it trains the immune system to recognize harmful germs without making us sick.
Layered defense
The immune system protects the body from infection using several layers of defense. The first layer includes physical barriers that stop bacteria and viruses from entering. If a germ gets past these barriers, the innate immune system helps to fight it. This system works quickly but doesn’t know exactly what it’s fighting.
If the germ avoids the innate immune system, vertebrates—like humans—have another layer called the adaptive immune system. This system learns to recognize the germ better each time it attacks. After the germ is gone, the adaptive immune system remembers it, so it can fight back faster if the same germ tries to infect again. Both parts of the immune system can tell the difference between the body’s own molecules and foreign substances.
| Innate immune system | Adaptive immune system |
|---|---|
| Response is non-specific | Pathogen and antigen specific response |
| Exposure leads to immediate maximal response | Lag time between exposure and maximal response |
| Cell-mediated and humoral components | Cell-mediated and humoral components |
| No immunological memory | Exposure leads to immunological memory |
| Found in nearly all forms of life | Found only in jawed vertebrates |
Surface barriers
Our bodies have special defenses to keep germs out. Mechanical barriers like skin and the cuticle of leaves help block germs. When we cough or sneeze, we push germs out of our lungs. Tears, mucus, and urine also wash away germs.
Chemical barriers include special substances in our body fluids. For example, enzymes in saliva and tears can fight bacteria. The stomach uses gastric acid to destroy harmful germs we might swallow. Good bacteria in our intestines also help by competing with bad bacteria.
Innate immune system
Further information: Innate immune system
The innate immune system is the body's first way to fight off germs. It acts fast when the body is attacked by things like viruses and bacteria. This system works in a general way and does not remember the germs like another part of the immune system does.
Cells in the innate immune system can find signs that something is wrong inside the body. They look for special pieces that many germs have or that come from cells that are hurt. This system has many types of cells, such as macrophages, neutrophils, and dendritic cells. These cells can catch and destroy pathogens. Inflammation is a response that makes more blood and immune cells go to the problem area. This can make the area red, swollen, hot, and sore. The complement system also helps by attacking foreign cells and pointing them out so other immune cells can destroy them.
Adaptive immune system
Further information: Adaptive immune system
The adaptive immune system helps the body fight infections better and remembers invaders from the past. It recognizes tiny parts of germs called antigens. When it finds these antigens, it makes special cells called memory cells that remember the invader. If the same germ tries to infect the body again, these memory cells act quickly.
The adaptive immune system has two main types of cells: B cells and T cells. B cells make proteins called antibodies that can stop germs. T cells come in different kinds, such as helper T cells that guide the immune response and killer T cells that remove infected cells. Together, these cells protect the body from harmful germs and remember them for future protection.
Physiological regulation
The immune system helps control many body functions. It works with other body systems, like the endocrine and nervous systems. It is important for babies as they grow, for fixing injuries, and for healing wounds.
Hormones can change how the immune system works. For example, female sex hormones can make the immune system more active. Male sex hormones like testosterone can make it less active. Sleep also affects the immune system. Not enough sleep can make it weaker. Good sleep helps the body fight off germs. Exercise usually helps the immune system. Very hard workouts might make it weaker for a short time, but it returns to normal later. The immune system also helps repair and heal damaged tissues in the body.
Disorders of human immunity
The immune system can sometimes fail to protect the body properly. This can cause three main problems: immunodeficiencies, autoimmunity, and hypersensitivities.
Immunodeficiencies happen when parts of the immune system don't work well. This can be because of age, poor diet, or inherited conditions. For example, some people are born with immune systems that don't develop fully. This makes them more likely to get infections.
Autoimmunity happens when the immune system mistakenly attacks the body’s own cells. This can lead to diseases like rheumatoid arthritis or type 1 diabetes. Normally, the immune system learns to recognize the body’s own cells, but sometimes this process fails.
Manipulation in medicine
The immune system can be helped by medicines. Doctors use special drugs to calm down an overactive immune system, like in allergies or after someone gets a new organ. These drugs can have side effects, so they are used carefully.
Vaccines are another way to help the immune system. They give it a tiny bit of a germ, like a virus, so the body can learn to fight it without getting sick. This is how we stay safe from diseases like measles and polio. The immune system also helps fight cancer by spotting and attacking unusual cells in the body. Sometimes, cancer cells can hide from the immune system, but doctors are finding new ways to train the immune system to attack these cells too.
Evolution and other mechanisms
Further information: Innate immune system § Beyond vertebrates
The immune system helps protect living things from diseases. Simple life forms like bacteria have ways to fight off viruses. More complex animals have advanced systems. For example, many animals use special proteins to spot harmful germs, and plants send out signals to protect themselves when they get sick.
Some germs have ways to hide from the immune system. They might change their surface or hide inside cells to avoid being found. These tricks help them stay inside their host.
History of immunology
Further information: History of immunology
Immunology is the study of how the body protects itself from sickness. People have known about immunity for a very long time. Over 2,000 years ago, during the plague of Athens, people noticed that those who had gotten better from the sickness could help others without getting sick again. In the 10th century, a doctor named al-Razi discovered that having smallpox once made a person safe from it in the future. Much later, Louis Pasteur used these ideas to create vaccinations and showed that tiny germs cause disease. His work was proven by Robert Koch, who confirmed that microorganisms make us sick.
By the late 1800s, scientists like Paul Ehrlich and Elie Metchnikoff made big steps in learning how the body fights off infections. Their important work helped us understand how our immune system works to keep us healthy.
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