Innate immunity refers to those characteristics of immunity that are present at birth and passed down genetically from parents to children. It consists not only of the skin and mucus membranes, but of the blood cells responsible for fighting off pathogens like bacteria and viruses. Innate immunity is different from acquired immunity, which is not present at birth, and which occurs when the immune system fights off a pathogen and creates antibodies against that pathogen to provide future immunity.
The first major element of the innate immune system is the skin. Most pathogens cannot penetrate the skin, unless the integrity of the barrier has been compromised by injury. Ciliary action in the lungs and nasal passages helps to expel pathogens from the body before they can cause disease. Tears, saliva, urine, and the sloughing of dead skin cells are all innate immune functions that help keep pathogens from infecting the body. The mucosal lining of the digestive and respiratory tract also traps pathogens before they have a chance to take hold in the body and cause disease.
White blood cells are one of innate immunity's most important features. The different types of white blood cells include phagocytes, macrophages, and natural killer cells. White blood cells serve the immune system by identifying and eliminating pathogenic threats. These cells kill bacteria and viruses.
The different types of white blood cells have different roles in the innate immune system. Macrophages develop when monocytes, a type of white blood cell that moves into infected tissues, enlarge and become filled with enzymes that help them eliminate bacteria. Macrophages remain in the tissues at the site of infection, removing bacteria and damaged dead cells.
Other white blood cells of the immune system have varying abilities to trap, kill, ingest, and digest pathogens. Basophils, a type of white blood cell implicated in allergic reactions, release histamine when they encounter allergens. Histamine increases blood flow to infected tissues, allowing other white blood cells more opportunity to remove invading pathogens. Dendritic cells, found in the lymph nodes and skin, help break down antigens so that T-cells can recognize and eliminate them. Natural killer cells can eliminate viruses by killing the cells that they infect.
Innate immunity comes with the built-in ability to form new immunities to specific pathogens. The white blood cells and blood proteins of the innate immune system are responsible for the body's ability to acquire immunity. Acquired immunity occurs when white blood cells create antibodies against a particular pathogen, rendering the individual immune to that pathogen's disease in the future. Acquired immunity is often induced through the use of vaccinations.