Immune Response Steps Part 1: The Innate Immune System
Innate / Non-Specific Immunity
We're all born with our first line of defense, the innate immune system. Due to innate immunity, humans aren't susceptible to many diseases that affect other animals and vice versa. The innate immune system involves non-specific, global defensive measures that take effect as soon as a pathogen is detected in the body that involves both mechanical and chemical factors.
The body has many mechanical means of defense. First and foremost is the skin. The skin is the largest organ in the human body and also the first protective barrier that keeps microorganisms from entering the body. Mucous in the nose, throat, and gastrointestinal tract trap pathogens and force them out before they can infect the lungs or intestines. Finally, the flushing of tears and saliva keep the eyes and mouth free of most potential invaders.
When mere physical impediments fail to do the trick, the body has a whole arsenal of chemical safeguards available as well. The acids found in sweat make conditions difficult for bacteria to grow. Certain enzymes in tears, saliva, and nasal secretions can break down the cell walls of bacteria and destroy their membranes. Proteins in the lung and gastrointestinal tract have antimicrobial properties. The lungs have further protection in the form of phagocytosis promoting compounds. Phagocytosis is the process, carried out by specialized cells, of swallowing harmful particles.
Innate Immunity inside the Body
If the body is breached, pathogens can enter and infection occurs. When this happens, the body's response becomes more enegetic. First, the complement system is activated. The complement system consists of circulating proteins. Most of the time they lie inactive, but when there is an infection, they become active. They can increase the permeability of the vascular system, thereby allowing components of the immune system better access to the pathogen. They also begin releasing cytokines (the immune system's couriers) that bring phagocytes to the scene. Finally, once the adaptive immune response has gotten involved, they begin working with specific antibodies to destroy pathogens. The coagulation system is sometimes also activated. When platelets coagulate, they release certain compounds that aid the immune system. In particular, they increase vascular permeability and attract phagocytes. Some of these compounds are also antimicrobial.
Other proteins and enzymes circulating in the blood also play a part in these later stages of the innate immune response. Proteins known as interferons limit the ability of viruses to replicate inside cells. Two other proteins, lactoferrin and transferrin, bind to the iron which bacteria need to grow. An enzymes called lysozyme breaks down bacterial cell walls. Finally, Interleukin-1 causes fever. Many viruses and bacteria cannot survive at higher temperatures.
The Cells of Innate Immunity
The last part of the innate immune system involves the cells capable of killing bacteria and parasites and destroying viruses. Neutrophils and macrophages phagocytize invaders. Natural killer cells indiscriminately destroy cells infected with viruses and that might be tumorous. Eosinophils can kill some parasites. These cells, while not specifically targeted, can be quite effective. Unfortunately, because they are not tailored to a specific pathogen, the body also suffers quite a lot of collateral damage. Many of the symptoms of common infections are not caused by the pathogen per se, but rather, the body's wholesale response to these invaders. Fortunately, the body does have another way of dealing with pathogens: the adaptive immune system.