Ever wondered why do we not get Chicken Pox twice? Ever thought why are vaccines pathogens in themselves? Ever wondered why should we not mix certain blood groups? The answer to all these depend on one fundamental entity or rather a gift of nature to all organisms - Antibodies. The response of immune system to external entities like virus, bacteria, fungi and other microbes that cause diseases in organisms is in creating complementary structures that either kill or inhibits them are Antibodies.