Human health is based on a powerful mechanism, our immune system. The ability of the human body to fight diseases and heal itself is one of nature's great mysteries and miracles. In recent years, however, scientific research has revealed that this ability is due to the action of an extensive network of specialized cells, regulatory proteins, and genes with specific purposes, all together forming a complex mechanism that operates continuously. The new knowledge inevitably gave rise to ideas on how we could influence this remarkable mechanism; thus, new types of drugs and specialized therapies, called immunotherapies, gradually emerged, helping us fight cancer, diabetes, arthritis, and a range of age-related diseases.