An injection containing a weakened form of disease-causing organism will usually trigger

A vaccine contains half killed or weakened pathogen such as a virus or bacterium, which causes a specific disease. A vaccine can be administered orally or through an injection in a healthy human. The function of the vaccine is to train the immune system by detecting the pathogen and kill them. After vaccination the cells (B lymphocytes) of the immune detects the antigens in the vaccine. The B lymphocytes make antibodies against the antigen and eliminate antigens. These antibodies  remained in the body and in case of real infection, they protect our body from pathogen.