Antibody/ Antigen?

An ANTIGEN is any molecule that can elicit an immune response--it can be a protein, a lipid, a carbohydrate, almost anything. It could be one that is foreign to the body or it could even be a host's own molecule under certain conditions. But a typical immune response is usually mounted against antigens from a foreign invasion, e.g. bacterial or viral infection. In that context, an antigen can be seen as a marker for the "bad guys."

An ANTIBODY is a protein molecule produced and secreted by B lymphocytes during an immune response; it circulates around in the blood, and is capable of recognizing and binding to a specific antigen. In that situation, think of an antibody as having been produced by the "good guys." If the antibody binds to an antigen that happens to be on the surface of say, an invading bacterium or virus, its binding serves as a molecular "red flag" which can be easily recognized and gobbled up by phagocytic cells (e.g. macrophages) of the innate immune system. The whole mess--bacterium/antigen-antibody--gets gobbled up and destroyed.

So, bottom line is, an ANTIGEN is a molecule, often one from a foreign invader. An ANTIBODY is another molecule that can specifically recognize and bind to an antigen. Think of them as being analogous to a lock & key.