An important component of the immune system, antibodies are protein molecules made by B cells. The antibodies circulate in the bloodstream, attaching themselves to pathogens in order to identify invaders. Immune cells responsible for killing invading organisms can then recognize pathogens, and destroy them, in response to attached antibodies. While circulating antibodies certainly help the immune system to eradicate infection quickly, they don’t circulate all the time. Instead, they’re produced only as needed, in response to one of a number of signals of infection.