The innate immune system is the body’s first line of defense against infection by microorganisms such as viruses, bacteria, and parasites. This branch of the immune system is referred to as innate because it is activated immediately upon infection to defend against all pathogens. In contrast, the acquired immune system, the body’s second line of defense, is a much more specific reaction that takes time to build. The innate immune system consists of a multi-pronged attack which includes physical barriers to infection, immune cells, and protein molecules called complement and cytokines.
Of the physical barriers to infection, the most important is the skin. When intact, the skin is impervious to most invading pathogens, and additional mechanisms such as sweating help to flush bacteria and viruses from the skin. Similarly tears, mucus, and saliva flush pathogens from the eyes and nasopharynx. The gastrointestinal tract is also part of the innate immune system. Defense mechanisms in this location include the acidity of the tract, enzymes that digest organic matter, and antibacterial proteins called defensins.
When an infectious agent is able to bypass or negate these physical barriers, the first reaction of the innate immune system is an inflammatory response. This response is stimulated by the release of inflammatory chemicals by cells that are injured or dying. Inflammation can be triggered not only by infection, but also by injury. When infection is present, however, additional defense mechanisms are triggered in conjunction with the inflammation. These include both cellular and chemical responses.
The presence of pathogens at a site of inflammation triggers the release of a large number of different chemicals, some of which simply promote inflammation and some of which have other roles. In response to the presence of bacterial proteins, a chemical cascade called the complement system is triggered. This is a chain of chemical reactions involving a large number of different proteins. The completion of the complement cascade helps kill invading bacteria, and also recruits more immune cells to the infection site.
Other substances, such as lactoferrin, transferrin, and lysozyme, are also produced at the infection site. Lactoferrin and transferrin bind available iron to limit its availability to bacteria, while lysozyme helps destroy bacteria by breaking down their cell walls. Immune system-specific chemicals called cytokines are produced as well. These include interferons, which help reduce the rate of viral replication, and interleukin-1, which increases the effectiveness of the complement reaction.
While these chemical responses are developing, the cells of the innate immune system, including neutrophils, macrophages, and natural killer cells, also have their own parts to play. These cells are recruited to the site of infection by cytokines and other inflammation-promoting proteins. All three cell types fight invading pathogens, but do so via different mechanisms. Neutrophils and macrophages kill by engulfing bacterial cells and digesting them, while natural kill cells destroy cells that have been infected by viruses. Macrophages are also involved in triggering acquired immunity, which develops in response to continued infection.