The immune system is a system of many biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organisms own healthy tissue. Your immune system does a remarkable job of defending you against disease-causing microorganisms. The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. One of the important cells involved are white blood cells, also called leukocytes, which come in two basic types that combine to seek out and destroy disease-causing organisms or substances. The immune system is divided into those which are static, or innate to the organism, and those which are responsive, or adaptive to a potential pathogen or foreign substance.