The immune system is a complex network of cells, tissues, and organs that work together to identify and eliminate harmful pathogens while sparing normal, healthy cells. The immune system has evolved to distinguish between self and non-self antigens and employs a variety of cells, including white blood cells, and molecules, such as antibodies and cytokines, to mount an attack against invading pathogens. The immune response is tightly regulated to prevent damage to normal, healthy tissue. Dysfunction in the immune system can lead to autoimmune disorders or immunodeficiency diseases.