The immune system is a network of cells and organs that work together to protect the body from infectious organisms. Many different types of organisms such as bacteria, viruses, fungi, and parasites are capable of entering the human body and causing disease. It is the immune system's job to recognize these agents as foreign and destroy them.

The immune system can respond to the presence of a foreign agent in one of two ways. It can either produce soluble proteins called antibodies, which can bind to the foreign agent and mark them for destruction by other cells. This type of response is called a humoral response or an antibody response. Alternately, the immune system can mount a cell-mediated immune response. This involves the production of special cells that can react with the foreign agent. The reacting cell can either destroy the foreign agents, or...