A virus hijacks the resources of a host cell to create copies of itself. These copies can then then move into nearby cells and hijack those too, setting off a

A virus hijacks the resources of a host cell to create copies of itself. These copies can then then move into nearby cells and hijack those too, setting off a biological chain reaction. An antibody is just a protein molecule with no ability to create copies of itself. It might move from one cell to another, but that does create a new synthesizer of the antibody at the new location, and so does not trigger a spread of the antibody throughout the tissue or organism.