Innate immunity refers to those characteristics of immunity that are present at birth and are genetically transmitted from parents to children. It is made up of not only the skin and mucous membranes, but also the blood cells responsible for fighting pathogens like bacteria and viruses. Innate immunity is different from acquired immunity, which is not present at birth and occurs when the immune system fights a pathogen and creates antibodies against that pathogen to provide future immunity.
The first important 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 flush pathogens out of the body before they can cause disease. Tears, saliva, urine, and the shedding of dead skin cells are innate immune functions that help prevent pathogens from infecting the body. The mucosal lining of the digestive and respiratory tracts also traps pathogens before they have a chance to take over the body and cause disease.
White blood cells are one of the most important features of innate immunity. 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.
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 fill with enzymes that help them kill bacteria. Macrophages remain in the tissues at the site of infection, removing bacteria and damaged dead cells.
Other white blood cells in the immune system have different 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 a better chance to clear invading pathogens. Dendritic cells, found in lymph nodes and skin, help break down antigens so that T cells can recognize and remove them. Natural killer cells can eliminate viruses by killing the cells 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, making the individual immune to disease from that pathogen in the future. Acquired immunity is often induced through the use of vaccines.