What is the name of a substance that stimulates the immune system to develop an acquired immunity?

A substance that stimulates the immune system to develop acquired immunity is called an "antigen." Antigens can be found in pathogens like viruses and bacteria, or in vaccines, which contain weakened or inactivated forms of these pathogens. When the immune system encounters an antigen, it triggers a response that leads to the production of antibodies, providing long-term immunity against future infections.