The answer is attach to cells with corresponding antigens.
Antibodies function in three distinct ways: they bind directly to antigens, effectively coating the surface of the invader, in order to prevent pathogens from entering or damaging the healthy cells; they can stimulate other parts of the immune system (e.g. complement proteins) to destroy pathogens; and antibodies can mark pathogens through opsonization so that pathogens can be identified and neutralized by other immune cells.
