mmune system is a powerful defense system which protect the host against invasion, growth and division of the infectious organisms that come from the out-side environment. It is consisted by two major systems: natural immunity, which constitutively protects the host against the invasion of microorganisms, and adaptive immunity, which is a highly developed system and can specifically recognize microorganisms and memorize them. These mechanisms of the immune system co-evolved with the microorganisms that invade them during the long history of the battle between them on the earth. As a result, the immune system developed elegant and sophisticated mechanisms to protect us from the invasion of the microorganisms. However, it is not perfect and is sometimes evaded by microorganisms that have developed varieties of the immune escape mechanisms. Understanding of these mechanisms should lead to the development of the novel therapies and vaccines against varieties of currently incurable infectious diseases.