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.
1 Antigens are proteins and other macromolecules that bind to a specific antibody and are used by the immune system to recognize pathogens.  B cells express receptors (BCRs) on their membrane which contain antibodies; these antibodies allow B cells to detect pathogens and release further antibodies to fight the infection.