In biology, immunity is the state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. It is the capability of the body to resist harmful microbes from entering it. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of wide range of pathogens irrespective of antigenic specificity. Other components of the immune system adapt themselves to each new disease encountered and are able to generate pathogen-specific immunity.
Innate immunity, or nonspecific immunity is the natural resistances with which a person is born. It provides resistances through several physical, chemical and cellular approaches. Microbes first encounter the epithelial layers, physical barriers that line skin and mucous membranes. Subsequent general defences include secreted chemical signals (cytokines), antimicrobial substances, fever, and phagocytic activity associated with the inflammatory responses. The phagocytes express cell surface receptors that can bind and respond to common molecular patterns expressed on the surface of invading microbes. Through these approaches, innate immunity can prevent the colonization, entry and spread of microbes.
Adaptive immunity is often sub-divided into two major types depending on how the immunity was introduced. Naturally acquired immunity occurs through contact with a disease causing agent, when the contact was not deliberate, whereas artificially acquired immunity develops only through deliberate actions such as vaccination. Both naturally and artificially acquired immunity can be further subdivided depending on whether immunity is induced in the host or passively transferred from an immune host. Passive immunity is acquired through transfer of antibodies or activated T-cells from an immune host, and is short lived—usually lasting only a few months—whereas active immunity is induced in the host itself by antigen and lasts much longer, sometimes lifelong. The diagram below summarizes these divisions of immunity.
A further subdivision of adaptive immunity is characterized by the cells involved; humoral immunity is the aspect of immunity that is mediated by secreted antibodies, whereas the protection provided by cell mediated immunity involves T-lymphocytes alone. Humoral immunity is active when the organism generates its own antibodies, and passive when antibodies are transferred between individuals. Similarly, cell mediated immunity is active when the organisms’ own T-cells are stimulated and passive when T cells come from another organism.
Immunity is a state in which the body is protected from infectious disease. It is conferred by the immune system, a complex network of cells, tissues and chemicals that fight infection and kill organisms when they invade the body. There are three categories of immune protection, all of which help protect the body from infectious diseases. It can be innate or acquired, active or passive, and natural or artificial. These categories can mix and match to produce, for example, natural passive or artificial passive immune protection.
The category of innate or acquired protection refers to the type of immune response that is mounted by the immune system. An innate immune response is not specific to the pathogen to which the system is responding, and it happens almost immediately when an infectious organism invades the body. In contrast, an acquired immune response is specific to the pathogen and can take several days to build up. The acquired immune response also involves the development of immunological memory, a state in which the immune system can quickly mount a response to an infectious organism that it has previously encountered.
Active or passive immune protection is determined by the way in which the protection is conferred. Protection that is active is conferred by contact with an infectious organism or a vaccine. This provokes an active immune response in the person who comes into contact with the organism. Passive immunity refers to the fact that an individual is protected, even though his or her immune system has not itself mounted a response. For example, the trans-placental transfer of antibodies from mother to child is a type of passive immune protection. Another example is the transfer of antibodies from mother to child in breast milk.
The third category, natural or artificial immunity, refers to whether the protection has developed with or without intervention. For example, trans-placental antibody transfer is a natural process, because it has occurred solely though an interaction between mother and fetus. If, after the baby was born, an antibody injection was administered, this would be an example of artificial protection, because the antibodies have been removed from one individual, purified, then injected into another. Vaccination is another example of artificial immune protection and is also an example of active acquired protection.
Vaccination and passive immune treatments are not the only ways to confer immune protection. It can be improved in many other ways, as has been demonstrated over the course of history. For example, improvements in sanitation, diet and pest control have contributed to the reduced severity of diseases and the increased life expectancy that people in developed countries enjoy now, compared to that of several hundred years ago.
What is immunity?
The immune system is your body’s way of helping to protect you from infection. When your body is infected by viruses, bacteria or other infectious organisms (e.g. a fungus or parasite), it undergoes a process of fighting the infection and then healing itself.
As a result of this, the next time your body encounters the same organism, you will be 'immune' to this infection. This means that you are less likely to get the same disease again, or if you do, the infection will be less severe. This is the principle behind vaccination.
How does immunity work?
Whenever your body encounters a foreign organism, like bacteria or a virus, a complicated set of responses are set in motion. Your body has two sets of defensive mechanisms, one called ‘innate immunity’ and another called ‘adaptive immunity’.
Innate immunity describes your body’s barriers to infection that are in-built (or innate). This includes:
- your skin
- the acid in your stomach
- mucus in your mouth and nose
- cells in your blood stream that can destroy bacteria.
All of these systems are extremely important as a first line of defence to prevent you from becoming infected, and for getting rid of the infections that you do get.
These innate systems do not change with multiple exposures to the same infection; there is no ‘learned’ response no matter how many times your body is exposed to the same organism.
Your body’s more complicated second line of defence is called adaptive immunity. By adaptingto fight infections from particular bacteria or viruses, your body can become immune to infections caused by the same organism in the future. This adaptation by your body to prevent infection is the basis of immunisation.
Certain types of blood cells can learn from exposure to an infection. This means that the next time they encounter that infection they can remember it and mount a faster and stronger response.
- antibodies are made by the body in response to an infecting organism. They can recognise specific types of viruses or bacteria. They work by attaching themselves to the organism, and preventing them from infecting your body.
- macrophages are specialised blood cells that can directly attack and destroy an infecting organism, digesting them so they can't produce disease.
Vaccines trigger the adaptive immune system — by stimulating the body to make antibodies — so that it can prepare for a potential infection in the future.