Once an antigen is noticed by the immune system, white blood cells called B-lymphocytes create a protein called an antibody that is designed to attach to that antigen. Many copies of this antibody are produced. If a true infection of the same disease occurs, still more antibodies are created, and as they attach to their targets they may block the activity of the virus or bacterial strain directly, thus fighting infection. In addition, once in place, the antibodies make it much easier for other parts of the immune system to recognize and destroy the invading agent.