Like all complex animals, the human body displays remarkable immune defenses against invaders, such as bacteria and virus, that would otherwise overwhelm the system.There are also internal parasites to be dealt with in each body. When a person sneezes, he or she expels thousands of possible pathogens and irritants. This is also true of every bodily function that takes anything from inside the body to the outside. In all bodily fluids and even in the sloughing off of the skin in microscopic flecks, microbes of many kinds are expelled from the body, moment to moment. Each human consists of a microbiome of both good, bad and mostly neutral "bugs."
The skin, the largest organ of the human body, is the first defense against invasion. When there is a scratch, nick or cut in the skin, that patch of skin is more vulnerable. The body reacts with inflammation and tenderness at the affected area. Cells that protect against infection multiply at the injury site. These remarkable defending cells can produce proteins which chemically seek out bacteria and even virus, targeting these invaders for destruction. Such white blood cells are a normal aspect of every immune system. They are produced at the rate of about 1,000 million per day within the bone marrow.
In the respiratory system are tiny hairs and mucous that trap invaders. Each inhalation delivers millions of microbes. They also come in the food we eat. The stomach produces acidic secretions such as hydrochloric acid which breaks down foreign material. Other organs assist with other protein attacking enzymes that are always ready to defend against pathogens.
Special T-cells and B-cells are produced within the body’s internal cells when a new and more serious illness threatens viral or bacterial infection. Highly specialized, T and B-cells are able to recognize a large variety of infectious agents. They also detect specific attacks with an encoded memory which provides the human body an acquired immunity, should the same type of attacking “germ” threaten the organism in the future. This is how a vaccination will provide an individual with a defensive army of agents, ready and able to defend when any such recognized threat surfaces again.
Most bacteria and fungi within the body of animals, including humans, co-exist in symbiotic and mutually beneficial roles. There are more non-human cells, about ten to one, than there are human cells in each person. They perform a great number of functions from digesting food to creating vitamins. They also play important parts in aiding the immune system.
An allergy develops when defensive cells identify something non-threatening as an invader. Pollen and certain foods trigger the immune system to defend itself against something it sees as unfamiliar. The same kind of over-reaction may occur in autoimmune diseases where human cells are effectively attacking other human cells.
These reactions, as well as reactions to such things as donated tissue, organs or blood must be monitored carefully. The body is extremely intelligent about identifying the unfamiliar, but it cannot always know what agents are attackers and which are not, with one hundred percent accuracy.
When one considers just how much of every organism is linked to every other organism both internally and externally, it is remarkable that the living self has any sense of being an individual. Like the planet and biological systems inter-connected and interdependent, each organism is a constellation of many interactive organisms. Each is actively engaged in the business of life at each successive heartbeat and breath.