The principal function of immune defense is protection against infection and invasion of the body space by foreign substances of all kinds. When we are ill with a viral infection such as a cold, we expect to get better, as a result of successful immune defense strategies. Another role of the immune system is the defense against molecules which invade body space from the outside. Immune cells are found circulating in the blood stream or patrolling in the intercellular spaces of all body tissues. Immune networks are also distributed in discrete organs of the lymphatic system which includes lymph nodes, tonsils, liver and spleen.
Bone marrow is the major manufacturing area of immune cells.
The thymus is the organ which originates one of two populations of immune cells, T-lymphocytes , at least in early life. In later life the thymus continues to exert regulatory influences on the immune system. Immunity means that immune cells remember the identity of an antigen challenge and initiate a successful defensive response. Immunizing injections contain antigens which belong to the infecting organisms. These immune clones can later identify and attack the infecting organism.