SafekipediaThe immune system is a network of biological systems that protects an organism from diseases. It finds and stops many kinds of pathogens, such as viruses, bacteria, and parasites. It also fights cancer cells and foreign things like wood splinters.
The immune system has two main parts: the innate immune system, which gives a general response to danger, and the adaptive immune system, which learns to recognize specific threats and remembers them.
Nearly all living things have some kind of immune defense. Even simple bacteria have tools that fight off viral infections. More complex animals, including humans, have advanced immune systems. One special feature is adaptive immunity. This helps the body get better at defending itself each time it faces a germ. That is why vaccination works—it teaches the immune system to recognize harmful germs without making the person very sick.
When the immune system does not work right, it can cause health problems. Some people are born with or develop conditions that make their immune systems weaker, called immunodeficiency. This can make them more likely to get infections. Sometimes, the immune system can become too active and attack the body’s own tissues, leading to autoimmune diseases. Learning about the immune system is important for understanding and treating many diseases, and it is the focus of the science of immunology.
Layered defense
The immune system helps keep our bodies safe from infection. It uses different layers to protect us. The first layer is our skin and mucus. These act like barriers to stop bacteria and viruses from getting in.
If a germ gets past these barriers, the innate immune system steps in. This system works very fast but does not target one specific germ.
If germs still get through, humans and other animals have another defense called the adaptive immune system. This system learns to recognize the germ and remembers it. The next time the same germ tries to attack, the immune system responds faster and stronger. The immune system can tell the difference between our own cells and substances that do not belong, which helps it know what to fight.
| 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 our skin, the cuticle on leaves, and the exoskeleton of insects help protect us. 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 saliva, tears, and breast milk that can stop germs from growing. The stomach uses strong gastric acid to break down harmful things we might swallow. Friendly germs in our intestines also help by using up food and space that harmful germs could use.
Innate immune system
Further information: Innate immune system
The innate immune system is the body's first way to fight off germs like viruses, bacteria, and parasites. It acts fast to stop infections but does not remember germs like the adaptive immune system. This system is important for all living things, including plants.
Cells in the innate immune system can tell when something is wrong, like when a germ enters the body or when a body cell is hurt. They use special proteins called pattern recognition receptors to find these dangers. Some important cells in this system are macrophages, neutrophils, and dendritic cells, which can find and destroy germs. When tissues are infected or hurt, the body may react with inflammation, which can cause redness, swelling, heat, and pain — signs that the immune system is helping to heal the area.
Adaptive immune system
Further information: Adaptive immune system
The adaptive immune system helps our bodies fight germs better each time we get sick. It "remembers" past infections and can act faster if the same germ tries to infect us again. This system works by recognizing special parts of germs called antigens.
Special cells in our body called lymphocytes help with this process. There are two main types: B cells and T cells. B cells make proteins called antibodies that can stick to germs and mark them for destruction. T cells come in different kinds too—some help other cells fight germs, while others can directly kill infected cells. When these cells spot a germ they've seen before, they can quickly act, making the illness milder or even stopping it before we get sick.
Physiological regulation
The immune system works with other body systems, such as the endocrine and nervous systems. It helps babies grow, fixes damaged tissues, and regrows lost parts.
Hormones can change how the immune system works. Sleep is very important for the immune system — not enough sleep can make it weaker. Exercise helps the immune system, but very hard exercise might make it weaker for a short time before it gets better. The immune system also helps heal and fix tissues in the body after an injury.
Disorders of human immunity
The immune system can sometimes have trouble working right, which can cause health problems.
Immunodeficiencies happen when parts of the immune system don’t work well. This can be because of age, not eating enough good food, or conditions a person is born with. For example, some people are born with conditions that make it hard for their bodies to fight off infections.
Autoimmunity happens when the immune system mistakenly attacks the body’s own tissues. This can lead to diseases like rheumatoid arthritis and diabetes mellitus type 1.
Hypersensitivity is when the immune system reacts too strongly to things that aren’t harmful, like pollen or certain foods. This can cause allergies and other reactions.
Inflammation is the body’s natural way of responding to infection or injury, but sometimes it can happen without a clear reason. This involves chemicals that call immune cells to the area and help heal any damage.
Manipulation in medicine
The immune system can be helped or calmed to keep people healthy. Doctors can calm an overactive immune system with special medicines called immunosuppressive drugs. These medicines stop the immune system from attacking the body’s own tissues. After an organ transplant, these drugs help prevent the new organ from being rejected.
We can also help the immune system fight bad germs through vaccination. Vaccines give the immune system a tiny, safe piece of a germ. This teaches it to recognize and fight the real germ if it ever shows up. This is why vaccines are important—they help protect people from diseases without making them sick first.
Evolution and other mechanisms
Further information: Innate immune system § Beyond vertebrates
The immune system has changed over time in many kinds of living things to protect against germs and other harmful agents. In very simple organisms like bacteria, there are special ways to fight off viruses. For example, bacteria can use a system called the restriction modification system to defend themselves from viral infections known as bacteriophages. Even simpler life forms have ways to recognize and fight off threats.
In more complex animals, the immune system includes special proteins that can recognize harmful germs. Animals and plants share some basic defense tools, like antimicrobial peptides called defensins. Many creatures also use a system called the complement system and cells that can swallow up invaders. Plants, which cannot move, use chemical signals to warn other parts of the plant when one part is under attack, helping the whole plant become more resistant to disease.
History of immunology
Further information: History of immunology
Immunology is the study of how our bodies stay healthy and fight off sickness. People have known about this for a very long time. During the plague of Athens in ancient Greece, people who had gotten sick before could help others without getting sick again. Later, a doctor named al-Razi noticed that people who had smallpox did not get it again.
In the 1800s, Louis Pasteur created vaccination and showed that tiny germs can make us sick. This was proven by Robert Koch, who showed that tiny living things cause infectious disease. Scientists like Paul Ehrlich and Elie Metchnikoff later learned more about how our bodies fight off sickness.
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