Immune system is the body’s defense (army) against infectious organisms and other invaders. In most cases, the immune system does a great job of keeping people healthy and preventing infections. In other words we may call it “the natural doctor”. Human body is the best doctor (in the world) if the immune system is healthy and fully functional. But sometimes problems with the immune system can lead to illness and infection.
The immune system is made up of a network of special cells, proteins, tissues, and organs that work together to protect the body. One of the important cells involved are white blood cells, also called leukocytes, which come in two basic types that combine to seek out and destroy disease-causing organisms or substances. These are phagocytes, cells that chew up invading organisms and lymphocytes, cells that allow the body to remember and recognize previous invaders and help the body destroy them.
There are two kinds of lymphocytes – B lymphocytes and T lymphocytes. They have separate functions: B-lymphocytes are like the body’s military intelligence system, seeking out their targets and sending defenses to lock onto them. T- lymphocytes are like the soldiers, destroying the invaders that the intelligence system has identified.
All of these specialized cells and parts of the immune system offer the body protection against disease. This protection is called immunity.
Humans have three types of immunity — innate, adaptive, and passive:
Everyone is born with innate (or natural) immunity, a type of general protection. Innate immunity also includes the external barriers of the body, like the skin and mucous membranes (like those that line the nose, throat, and gastrointestinal tract), which are the first line of defense in preventing diseases from entering the body. If this outer defensive wall is broken (as through a cut), the skin attempts to heal the break quickly and special immune cells on the skin attack invading germs.
The second kind of protection is adaptive (or active) immunity, which develops throughout our lives. Adaptive immunity involves the lymphocytes and develops as people are exposed to diseases or immunized against diseases through vaccination.
Passive immunity is “borrowed” from another source and it lasts for a short time. For example, antibodies in a mother’s breast milk give a baby temporary immunity to diseases the mother has been exposed to. This can help protect the baby against infection during the early years of childhood.
Everyone’s immune system is different. Some people never seem to get infections, whereas others seem to be sick all the time. As people get older, they usually become immune to more germs as the immune system comes into contact with more and more of them. That’s why adults and teens tend to get fewer colds than kids — their bodies have learned to recognize and immediately attack many of the viruses that cause colds.