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The immune system is the body's defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms and substances that invade body systems and cause disease. The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. The cells involved are white blood cells, or leukocytes, which come in two basic types that combine to seek out and destroy disease-causing organisms or substances. Leukocytes are produced or stored in many locations in the body, including the thymus, spleen, and bone marrow. For this reason, they're called the lymphoid organs.

There are also clumps of lymphoid tissue throughout the body, primarily as lymph nodes, that house the leukocytes. The two basic types of leukocytes are: phagocytes, cells that chew up invading organisms. lymphocytes, cells that allow the body to remember and recognize previous invaders and help the body destroy them. When antigens (foreign substances that invade the body) are detected, several types of cells work together to recognize them and respond. These cells trigger the B lymphocytes to produce antibodies, specialized proteins that lock onto specific antigens. Once produced, these antibodies continue to exist in a person's body, so that if the same antigen is presented to the immune system again, the antibodies are already there to do their job. 

 Immunity 

Humans have three types of immunity — innate, adaptive, and passive:

Innate Immunit: Everyone is born with innate (or natural) immunity, a type of general protection.

Adaptive Immunit:The second kind of protection is adaptive (or active) immunity, which develops throughout our lives.

Passive Immunit: Passive immunity is "borrowed" from another source and it lasts for a short time. For example, antibodies in a mother's breast milk provide a baby with temporary immunity to diseases the mother has been exposed to.

The immune system

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