Antibody

Antibodies are defense proteins made by cells called B cells. Their other name, the one used in the lab, is immunoglobulins: it's the same thing said with the technical word, not two different molecules.

What makes an antibody work against one thing and not another is its hypervariable regions, stretches of the molecule that vary enormously from one antibody to the next. Those stretches are what build the antigen-binding site, the pocket where whatever the antibody recognizes fits in. It's the logic of a key: the body can look much the same from one key to the next, but the pattern cut into the tip is what decides which door it opens.