The immune system is an intricate network of organs, highly specialized cells, and chemical signals that work to protect our bodies from foreign invaders, particularly infections. The immune system is able to differentiate between normal host cells and foreign cells because different types of cells frequently display different antigens. Antigens are proteins that are found on the surface of cells and often differ from one type of cell to the next. While normal, host cells also present antigens, the immune system is able to differentiate between antigens found naturally within the body (self-antigens) and antigens from foreign invaders.
When the immune system encounters a foreign antigen, a cascade of events occurs which is intended to clear the foreign substance from the body. Initially, foreign cells are encountered by antigen-presenting cells (APCs, the most important of which is the dendritic cell), which engulf them and break up the foreign antigen into small protein pieces known as epitopes. The APC presents these epitopes to lymphocytes, a type of white blood cell. There are two main types of lymphocytes: T-cell lymphocytes and B-cell lymphocytes. The APC can present the epitope to either a CD8 (cytotoxic) T-cell which go on to directly kill the foreign cells, or to a CD4 (helper) T-cell which release chemical signals that work to both help cytotoxic T-cells kill foreign cells and to induce B-cell lymphocytes to produce antibodies.
The presentation of antigens to T-cells is not enough to produce a vigorous immune reaction. Co-stimulation through a variety of chemical signals must also occur in order for the immune response to adequately respond. Without these co-stimulatory signals, the response of T-cells is weak and often inadequate at destroying the foreign cells. In the presence of co-stimulation, the reaction of T-cells and B-cells is brisk and additional cells, such as natural killer cells, are recruited to help destroy the foreign invaders. This complex system works together to form the immune response.