What's an antibody?
Antibodies are proteins made by cells in your immune system in response to specific foreign materials called antigens. They are part of your body's adaptive immune system. Antibodies are very specific: your body makes tons of antibodies, and each antibody binds, or sticks to, its particular antigen.
How and why does the immune system make antibodies?
The immune system exists to protect the body from dangerous things like bacteria and viruses. There are two divisions:
The innate or natural or nonspecific immunity. This includes physical barriers like your skin, nasty secretions like lysozyme, and cells called phagocytes that eat whatever comes their way. As the name suggests, you are born with these defenses in place and they are non-specific, that is, they work against anything that enters your body without regard for what it is.
the specific or acquired or adaptive immunity, which produces and uses antibodies, also called immunoglobulins. The purpose of an antibody is to bind to its very specific target, and alert other cells, called phagocytes (see above), to "eat" the target. The particular part of the target that the antibody binds to is called an antigen.
To give a very brief overview of how the adaptive immune response works, when an antigen (let's say a virus) gets into the body, a cell called an Antigen Presenting Cell (APC) will immediately "eat" (phagocytose) it, rip it up, and
"wear" parts of the virus on its surface, the better to present them to T cells. Next, a helper T cell will bring one of those antigen pieces to a B cell. Way back when the B cell was a little baby B-cell, it rearranged its DNA. The process is called VDJ recombination and is too complicated to explain here, but basically there is a great big antibody gene with lots of different parts to it that the cell can mix and match. There are so many parts, in fact, that there are 10^16 (that's 10,000,000,000,000,000, or ten quadrillion) possible ways to make an antibody. Each B cell makes one type of antibody, and wears a few on its surface.
So the helper T cell goes around with the antigen, showing it to all the B cells. Some of the B cells will be producing antibodies that can bind that particular antigen. The T cell will then "tell" those lucky B cells to make lots and lots of their antibodies, and to divide to make lots more B cells that produce the same antibodies.
All of those antibodies, meanwhile, go out through the bloodstream and stick to all the antigens they can find. All these antibodies, sticking to the virus (or whatever the antigen is) are like little "eat me!" signs for any phagocytes that come wandering by. A phagocyte can't tell a virus from a hole in the ground (phagocytes are actually part of the innate immunity, #1 above), but it knows what "eat me" means, and that's the end of our virus. Some of the B cells that helped out will stick around as memory cells, ready in case the antigen attacks again.
What does an antibody look like?
Your typical antibody is an IgG, but there are others in the immunoglobulin superfamily including IgM's, IgD's, IgA's, and IgE's. They all look more or less like this:
1 2 3 4
\ \ / /
\ \ / /
\ \ / / variable region
| | constant region
The parts in bold are the variable regions - these are the parts that result from the big mix-and-match. The rest of the antibody is the constant region, and is the same, or nearly the same, in all antibodies. An antibody is a protein, and so it is a great big string of amino acids. Notice that it is actually composed of four strings of amino acids (four polypeptides) - two long ones, called the heavy chains (numbers 2 and 3 in the diagram), and two short ones, called the light chains (numbers 1 and 4).
Antibodies for fun and profit: what antibodies can do for YOU.
Scientists don't have any machines or chemicals that can make antibodies from scratch; the only way to obtain them is from another animal, preferably a mammal like a rabbit, mouse, or horse (mice are easy to deal with in a lab whereas horses are less convenient but are large enough to produce marketable quantities of serum). For example, the venom of poisonous snakes is injected in small amounts into donor horses, and the horses make antibodies to it. Periodically blood is drawn and the antibodies are purified from a component of the blood called serum. Then when you go to the emergency room with snakebite, you're given antivenin -- antibodies that can tag the venom molecules for quick destruction by your immune system.
Antibodies also have a number of uses in the laboratory, which are too numerous to describe here. There are two ways of obtaining these: polyclonal antibodies are made in the same way as antivenin, and the desired antibody is purified chemically. A better technique is to use monoclonal antibodies: after injecting the mouse with antigen, B cells from its spleen are fused with immortal cells to make a cell line that produces only the one antibody you need (remember, each B cell only produces one type of antibody). This way you don't have to purify your desired antibody from a blood-and-antibody soup every time.
The antibodies thus obtained can be used in a number of ways in the laboratory. An exhaustive list of techniques is not within the scope of this node, but here are some examples: You can attach a radio label or fluorescent label to an antibody, so that it will attach itself (and thus the label) to the particular protein that it binds to (we'll call this your Protein of Interest, or POI). Then you can use special equipment to "see" the label. Or, there is immunoprecipitation: you add the anti-POI antibody to a mixture of molecules, and then separate the antibody-bound protein from the rest of the mix. (There is a lot more detail on this at the immunoprecipitation node).
That's really cool.
For more information, there is an excellent book - short, introductory, clearly written and well-illustrated - called Immunology: an Introduction for the Health Sciences by Seymour, Savage, and Walsh (1995, McGraw-Hill, Roseville Australia).