hen a bacterial infection is established in the body, the purpose of the immune system is to control or eradicate it. The initial reaction of the immune system to an infection varies, depending on the site which has been invaded and on the nature of the invader. There can be many "triggers", that can spur the immune system into action. Here are some of the ways in which the immune system can be activated.
If the invasion is in an area of the body that is primarily defended by macrophages, such as the lungs or intestines, then these macrophages will be the first immune cells on the scene. They begin to digest the invading organism, and by presenting antigens (proteins from the destroyed bacteria), they stimulate other cells of the immune system into action.
Some bacteria, for example Staphylococcus Aureus and Salmonella Typhi, produce chemotaxins when they enter the body, which betray their presence to the immune system, by acting as "breadcrumbs" which reveal the location of the invader. Chemotaxins are chemicals that activate phagocytes, the immune cells whose function it is to consume and destroy the invading bacteria.
Some bacteria first encounter, and are recognised by, the complement system, which in turn produces chemical messengers (cytokines) that warn other cells of the immune system that the body has been invaded.
The invader may be recognised by the acquired immune system, i.e. the lymphocytes. These cells either directly fight the infection themselves, or control other cells to do so.
