SafekipediaThe immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, such as viruses, bacteria, and parasites, as well as cancer cells and foreign objects, such as wood splinters—distinguishing them from the organism's own healthy tissue. This amazing system helps keep us healthy by fighting off germs and other harmful invaders.
Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions, working together to defend the body.
Nearly all organisms have some kind of immune system. Even bacteria have a rudimentary way to protect against viral infections. More complex animals, including humans, have a sophisticated defense mechanism. For example, adaptive immunity allows the body to learn and remember past invaders, making it better prepared if they return. This is why vaccination is so effective—it helps train the immune system to recognize harmful germs without causing disease.
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 kicks in. 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 line of defense against infections. It responds quickly to invaders like viruses, bacteria, and other harmful substances. This system works in a general way, not targeting specific enemies, and it does not create long-term protection like the adaptive immune system.
Cells in the innate immune system can recognize signs of infection or cell damage using special receptors. These receptors detect molecules common to many germs or pieces released by damaged cells. The system includes various cells, such as macrophages, neutrophils, and dendritic cells, which can engulf and destroy pathogens. Inflammation, a response that brings more blood and immune cells to the area, is an important part of this defense, causing redness, swelling, heat, and pain. The complement system also helps by attacking foreign cells directly and marking them for destruction by other immune cells.
Adaptive immune system
Further information: Adaptive immune system
The adaptive immune system helps the body fight off infections more effectively and remembers past invaders to respond faster next time. It works by recognizing specific parts of pathogens called antigens. When it finds these antigens, it creates special cells called memory cells that "remember" the invader. If the same pathogen tries to infect the body again, these memory cells quickly spring into action.
The adaptive immune system includes two main types of cells: B cells and T cells. B cells produce proteins called antibodies that can bind to and neutralize pathogens. T cells come in different kinds, such as helper T cells that direct the immune response and killer T cells that destroy infected cells. Together, these cells work to protect the body from harmful invaders and remember them for future defense.
Physiological regulation
The immune system helps manage many body functions and works closely with other body systems, like the endocrine and nervous systems. It is important for developing babies, fixing injuries, and healing wounds.
Hormones can change how the immune system works. For example, female sex hormones can boost immune activity, while male sex hormones like testosterone can reduce it. Sleep also affects the immune system. Not getting enough sleep can weaken it, while good sleep helps the body fight off germs. Exercise generally helps the immune system, though very intense workouts might temporarily lower it before it returns to normal. 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, leading to three main problems: immunodeficiencies, autoimmunity, and hypersensitivities.
Immunodeficiencies happen when parts of the immune system don't work well. This can occur because of age, poor diet, or inherited conditions. For example, some people are born with immune systems that don't develop fully, making them very susceptible to 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 or controlled by medicines. Doctors use special drugs to calm down an overactive immune system, like in allergies or when 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 has developed in living things to protect against diseases. Simple life forms like bacteria have ways to fight off viruses, and 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 part of them gets sick.
Some germs have learned how to hide from the immune system to survive inside their hosts. They might change their surface or hide inside host cells to avoid being detected. These tricks help them infect the host despite the body's defenses.
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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