What Is an Antibody?
Antibody molecules are wonderful things. These molecules are made of protein – meaning – the molecule is a bunch of different individual amino acids hooked together in a row (a polymer of amino acids). Each antibody molecule has two different polymers hooked together – one of these amino acid polymers is called a Heavy chain and the other one is called a Light chain. Guess why — you are correct – because one of these polymers is heavier than the other, or lighter than the other – depends on your perspective… See how precise science terms can sometimes be? Now, I am not going to call these things polymers any longer… the Heavy chain is a protein all by itself, as is the Light chain… and when the Heavy and Light chains are connected to one another (side-by-side), each folds around the other to form an HL pair.
Now, two pair of HL’s are _also_ connected to one another side-by-side to form the overall antibody molecule. So, there are at least two Heavy chains and two Light chains in an antibody molecule, and, _both_ Heavy chains within the same antibody molecule are absolutely identical to one another, and _both_ Light chains within the same antibody molecule are identical to one another.
Each Heavy chain is about 450 amino-acids long, and each Light chain is about 250 amino-acids long. Therefore, the Heavy chain weighs about 50,000 Daltons and the Light chain about 25,000 Daltons – IF – you had exactly 6.023 x 10 raised to the 23rd power (Avogodro’s number) – number of molecules of each of either Heavy or Light chains, respectively – that is – one mole of each. Therefore, one mole’s worth of a complete antibody would weigh about 150,000 Daltons. So, we say that the “weight” of an antibody is 150,000 Daltons. OK – what the heck is a Dalton? A Dalton is defined as one atomic mass unit – or – 1/12th of the mass of the most abundant isotope of the element, Carbon (a mass of 12.0000) However, if you look at a Periodic table of the Elements, you will see that the atomic weight of carbon is listed as 12.0111 – the reason being that this value is the average atomic weight of all of the known isotopes of Carbon — wait a minute – you don’t really care about all of this stuff, do you? Ha! Pardon me, I get carried away with these things, sometimes. No kidding; some folks would probably enjoy seeing me “carried away” . So, let’s talk about what these very fine molecules do for us….
The antibody molecule is a special kind of protein made by a cell of the immune system called the B-lymphocyte. Each B-cell produces antibody molecules which are different from the antibody molecules produced by every other B-cell – but – the difference is subtle – but critically important. Remember I said that the Heavy and Light chains fold around one another? — well — when this folding happens, a region of the molecule is formed which is capable of binding to molecular shapes which are present on other molecules – or on another antibody molecule for that matter.. This region is called the ANTIBODY COMBINING SITE – and, because there are two HL pairs, there are always at least two binding sites present on each and every antibody molecule. Because the H’s are identical to one another, and the L’s are identical to one another, _both_ binding sites on the same antibody molecule are identical to one another. This site is where things are bound to the antibody – very specifically. Each site “recognizes” a molecular shape. What I mean here is that because of the shape of the antibody binding site and the nature of the electron clouds generated by the different atoms within this site, the site can bind to other electron clouds – AS LONG AS THE ELECTRON CLOUD SHAPES “FIT” INTO THE SITE. If the shapes don’t fit, then the site will not bind the molecule.
If you will take a quick look at this image, you will see one of the binding sites of this protein – called Fab for Fragment, Antigen Binding – (there is always a minimum of two binding sites). Notice that the heavy chain is in blue, the light chain in green and the protein being bound is in red. If you stand and raise your arms above your head, you will “look” like an antibody. Your hands each represent a binding place – but your hands can bind only to one particular shape.
Now, antibody molecules made by B-lymphocytes circulate in the bloodstream and also in the lymph (the fluid in our tissue spaces).
Because of this circulation, antibody molecules generated in response to something foreign to us which has somehow gotten into our body – like a bacterium or virus – can appear anywhere throughout the body. If these circulating antibodies come in contact with the thing they were generated to appear against in the first place, then, the antibodies will bind to the foreign thing – the target. This binding will result in several possible outcomes – the target may be rendered inactive, it may be now more easily destroyed by some cells we have called macrophages (these cells “eat” things – _especially_ if things are coated with antibody molecules), or, the target may now be unable to associate with our tissue – the antibody molecules may cover the places on the target which allow association with our cells. Therefore, after awhile, a toxin, a virus, or a bacterium will no longer to be able to hurt us.
When a person gets immunized against something – like diphtheria toxin, pertussis toxin and tetanus toxin (the DPT shot little kids get and the tetanus shot we all continue – or should continue to get), or influenza virus (the flu shot), the thing that is trying to be accomplished is to activate a person’s immune system – and one important result of this activation is the organism-specific or foreign substance-specific response of our immune system cells – one of which is the B-cell. Therefore, we will make antibodies which can bind to the thing we are injected with. This response not only leads to an initial production of antibody, but also leads to more cells which “recognize” the foreign thing and which can make antibodies against it. After immunization – or after a natural recovery from some infection – we now have a bunch more cells specifically able to respond to the same foreign thing. So, by using dead bacteria, dead virus, or harmless forms of substances which these organisms produce (like tetanus toxin) as agents for immunization, we build up immunity potential. THEN, if the REAL substance or organism later enters our body, we can make a much, much better response – like many, many more specific antibodies for example – and, we might not even know that we have fought-off this danger.
Antibodies are also very important tools used in medicine and science.
Because of the ability of antibody molecules to very precisely “recognize” and bind to certain shapes on other molecules, we can use antibody binding activity to identify an organism – something really important sometimes in order to decide what kind of medical treatment we may need.
All in all then, antibody molecules are really nice proteins – just one of the many useful things our immune system does for us. You know, sometimes I just cannot believe how elegant all of these things are – amazing. We are very, very lucky.
Copyright John C. (Jack) Brown, January, 1996
Permission granted to
reprint to Vikki McInnis-Shaw