The term immunity comes from the Latin word immunitas, means protection from legal prosecution. Immunity refers to protection from disease and other pathogens. The cells and molecules responsible for immunity are called immune system and their efforts in regards to any etiological agent are called immune responses . Normally the immune responses are elicited against the foreign substances but occasionally to the self molecules and are referred as autoimmune responses. Immunology is a branch of life-science which deals with the cellular and molecular events occurring in the body after encounters of micro-organisms and other foreign substances.

The history of immunology is quite old. In ancient China, people often used skin lesions of patients recovered from small pox to cure small pox in young children. The first successful record of vaccination came from the work of Edward Jenner's efficacious vaccination against smallpox. Jenner observed that milkmaid who had recovered from cowpox never showed any symptom of smallpox. Following this observation he inoculated the cowpox pustules into the arm of a young boy who later did not show full progressive smallpox symptoms. Small pox was the first disease that was eradicated worldwide by vaccination.

Recently the science of immunology has grown up by the advent of new molecular biology tools. Our current understanding of the human and animal immune system and its functions has remarkably improved. Advances such as recombinant DNA technology, immunohistochemistry, monoclonal antibody production and x-ray crystallography have changed the immunology to a broader area. The development of techniques to produce transgenic and knockout mice has also played a great role to understand many complex immunological pathways.

1.1  Innate and adaptive immunity

Defense against microbes includes an early response action called innate immunity and a later response called as adaptive immunity. Innate immunity is also called natural or native immunity and provides first line of defense against any microbial infection in human body. It usually involves many cellular and biochemical events that react to microbes and their products in order to clear them from the body. The main components of innate immune system are

1)  Barriers – skin and outer epithelial surface.

2)  Scavenger cells – neutrophils, macrophages, dendritic cell and natural killer cells.

3)  Complement system

4)  Cytokines

5)  Chemical mediators of inflammation