Immunity is how your body protects itself against germs (like bacteria, viruses, and fungi) and toxins (chemicals made by germs). Your immune system has many organs, cells, and proteins that work together. It is made up of two different systems – innate immunity and acquired immunity.
The innate system is your first line of defense against infection. It is made up of a series of membrane-bound receptors and cytoplasmic proteins that recognize chemical structures that are characteristic of invading microbes but are absent from host cells. These recognition molecules are termed antigens. The innate response also includes soluble factors that activate other cells and promote inflammation (like complement proteins, defensins, ficolins, and cytokines).
A primary function of the innate immune system is to bind and destroy invading pathogens. It does this by recognizing specific parts of invading microbes called surface antigens. This allows leukocytes to enter the infected cell and remove the invading microbe. In addition, the innate system produces the powerful phagocytic cells neutrophils, which are abundant at sites of infection and tissue injury. They are important for clearing microbial pathogens and repairing damaged tissues. Neutrophils secrete large amounts of the inflammatory cytokines tumor necrosis factor and interleukin-12 as well as chemokines that attract other phagocytic cells.
The other main job of the immune system is to help prevent infection by recognizing and destroying germs before they cause illness. This is accomplished by recognizing the “antigens” that are characteristic of living cells, bacteria, viruses, and fungi, and by identifying and reacting to nonliving substances, such as toxins, chemicals, drugs, and foreign particles (like a splinter). The immune system also keeps records of every germ it has ever defeated in types of white blood cells called memory cells. This way, if that same germ invades the body again, it can be quickly eliminated.