Immunology is a branch of medicine studying our immune system at both molecular and clinical levels.  It deals with the physiological functioning of our immune system in both health and diseases, e.g. autoimmune diseases, hypersensitivities, immune deficiency, transplant rejection, etc.  Our immune system that protect our bodies against foreign insults and objects are sophisticated and involve many organs and subsystem, e.g. white blood cells, thymus, bone marrow, spleen, tonsils, lymph vessels, lymph nodes,adenoids, skin and liver.  Many components of the immune system are actually cellular in nature and not associated with any specific organ but rather are embedded or circulating in various tissues located throughout the body.