When an antibody attaches to a virus it prevents the virus to get in the other body cells. What happens next is that, the antibodies calls other immune agents to assist and take part to what is known as "plasma complement system".
First, the infectious virus/bacteria will be coated up by the antibodies and then the white blood cells engulf the virus/bacteria, destroy them and remove them from the body. It is the WBC that completes the job.
