Each Heavy chain is about 450 amino-acids long, and each Light chain is
about 250 amino-acids long. Therefore, the Heavy chain's mass is
about 50,000 Daltons per mole and the Light chain's mass is about 25,000
Daltons per mole. Therefore, 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, this amount of
molecules would have a mass of 150,000 Daltons. So, we say that the
"molecular mass" of this particular kind of antibody is 150,000
Daltons per mole - which is a mass of 150,000 grams per mole. 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 (which has a mass of 12.0000 grams per mole) However, if you look
at a Periodic Table of the Elements, you will see that the atomic mass of
carbon is listed as 12.0111 - the reason being that this value is the
average atomic mass 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"
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.
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. Please see: Some Good News
about a Bad Bug! which talks about an immunization which
protects us against the polio virus
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. Please see:
Mankato - A City Fights Back!. 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.
If you will take a quick look at the image to the left, you will see one of the
binding sites of an antibody molecule - named 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 (notice the image at the top of this page).
Your hands each represent a binding place - but your hands can bind only to
one particular shape.
Book: Don't Touch That Doorknob!
Copyright John C. (Jack) Brown, January, 1996