Two components, at least, should be considered to describe what we understand the immune system to be. The first consists of cells able to recognize strictly defined structures termed as antigens and to react to them in different specialized ways. The second comprises all cells and structures which possess the ability to attract immune recognition by the first. The union of both the components is based on the mutual complementarity of specialized structures described as antigens or receptors. Only direct contact of the opposite and complementary receptors can trigger immunity — the function indicative of the existence of the immune system.