What are antibodies? How do they  fight a bacterium?

Antibodies are native molecules produced by the body in response to the attack of antigens. These are also called immunoglobulins. These are specialised protein molecules which protect the body from all ill effects of the foreign bodies known as antigens. These are Y-shaped protein molecules composed of four smaller polypeptides linked together.
When an antigen enters the blood, it binds to a special type of white blood cell called lymphocyte. Each lymphocyte produces only one kind of antibody which binds to its surface. This attachment, in turn stimulates rapid division of cells producing daughter cells which are specialised in the production of that antibody. Thus, in response to the entry of antigen or infection the number of antibodies in the blood becomes much more than the antigens. As a result, a reserve supply is left in the body after antigens are destroyed. Newly born babies do not have a functioning system for antibodies synthesis, for several months. During this period, they are protected by antibodies from mother’s milk.