Antibodies are custom-made proteins created by the immune system to attack specific infections. They are primarily found in the blood, but can be found in any bodily fluid. They can both identify and neutralize foreign material in the body. They are produced by B-cells.
The general structure of antibodies is similar, but their tip is altered when they are created to fit to the protein coat of the particular infection they fight off. As such, they are able to surround the infection.