This chapter focuses on immunity and immunopathology including amyloidosis. An antigen (Ag) is defined as a substance, usually protein in nature, which when introduced into the tissues stimulates antibody production. Hapten is a non-protein substance which has no antigenic properties, but on combining with a protein can form a new antigen capable of forming antibodies. An antibody (Ab) is a protein substance produced as a result of antigenic stimulation. The antigen may induce specifically sensitized cells having the capacity to recognize, react and neutralize the injurious agent or organisms. Immunity may be natural or innate immunity or specific or adaptive immunity. Cells of immune system are Lymphocyte, B-cells, T-cells, T-helper cells and T-suppressor cells. Mast cells and basophils are thus involved in mediating inflammation in allergic reactions and have a role in wound healing. Eosinophils are circulating granulocytes and play a role in allergic reactions and in intestinal helminthiasis. Autoimmunity is a state in which the body’s immune system fails to distinguish between ‘self’ and ‘non-self’ and reacts by formation of auto-antibodies against one’s own tissue antigens. Amyloidosis is the term used for a group of diseases characterised by extracellular deposition of fibrillar proteinaceous substance called amyloid having common morphological appearance, staining properties and physical structure but with variable protein composition.