Early Indus Valley Civilization ... their Geograhpy, their customs and Indo-Aryan civilization with the earliest caste system.
Antibodies contribute to immunity in three main ways: they can prevent pathogens from entering or damaging cells by binding to them; they can stimulate removal of a pathogen by macrophages and other cells by coating the pathogen; and they can trigger direct pathogen destruction by stimulating other immune responses such as the complement pathway.
Infection with viruses or bacteria triggers the immune system to release proteins called antibodies. The specific part of the virus or bacterium that triggers the antibody response is called an antigen. Once produced by the body (specifically, by the lymphoid tissue, which is part of the body's immune system), antibodies circulate in the bloodstream until they come into contact with an antigen.