Immunology is a science that deals with the study of protection of the body from invasion by disease producing microorganisms. This state of protection of the body is called immunity. It is result of normally functioning immune system which is highly efficient but complex. This chapter covers the historical perspectives, structure and functions of immune system, lymphoid system, cells of immune system, macrophages, major histocompatibility, immune response, immunity, antigen, antibodies, enzyme-linked immunosorbent assay, lens proteins, human leukocyte antigens, and specialized proteins. Lymphoid system consists of lymphoid organs and lymphoid cells. Lymphoid organs are classified based on their function into central (primary) lymphoid organs and peripheral (secondary) lymphoid organs. Major histocompatibility (MHC) is also known as human leukocyte antigen (HLA) complex. It is a collection of genes arrayed within a long continuous stretch of DNA on chromosome 6 in humans.