B
cells (and T cells) are part of the adaptive immune response that
takes 4-7 days after infection to become effective. B cells are lymphocytes
or cells of the lymphatic system. B cells develop in the bone marrow,
hence their name. B cells produce antibodies. Each B cell produces
a specific antibody that will bind to a specific antigen (an antigen
is anything that triggers an immune response). For instance, one B
cell will produce an antibody that blocks a virus that causes the
common colds, while another will make an antibody that targets the
pathogenic bacteria that causes pneumonia.
The human body is constantly making new B cells that make antibodies
of different specificities in the hope at least one will be able to
stall the foreign pathogen. These different specificities arise from
the B cells reshuffling their DNA (De-oxyribose Nucleic Acid) to create
an enormous number of new genes from a limited pool of DNA. This process
is called DNA re-arrangement and is essentially a mix and match process
that gives rise to a very diverse spectrum of antibody genes. To give
you an idea of this diversity, there are about 1011 different antibodies
circulating in humans at a given time!
Antibodies are good sentinels against pathogens in the blood and body
fluids. Of all circulating antibodies, most have never seen their
specific antigen. However, there are also antibodies from 'memory'
B cells that have seen their specific antigen before. They are still
circulating in the blood after having destroyed the antigen, just
in case the pathogenic antigen comes back again. In that case, these
antibodies from memory B cells will be able to mount an effective
response in a much shorter time, which is the whole point of being
'immunized' against something. The immune system will be prepared
and able to clear the antigens in a shorter time with much less damage
to the body.
How Do Antibodies Work?
An antibody is essentially a large protein molecule that recognizes
and binds to a specific pathogen or antigen. An antibody recognizes
an antigen the way a key matches a lock; it may be quite precise,
or it may not be so precise, say like a skeleton key. The moment an
antibody binds to an antigen, it helps mark the antigen for subsequent
destruction. When the antibody binds to its specific antigen, the
B cell that made it becomes activated and begins to produce a lot
more antibodies of that specificity!
Antibody proteins are called immuno-globulins. At the molecular level,
the antibody protein is often depicted as a 'Y' shaped molecule with
two antigen-binding sites, one at the end of each 'Y' arm. The 'Y'
tail defines what class of antibody it is. You may have heard of gamma
globulin injections to fight off certain infectious diseases like
hepatitis. Gamma (G) globulins are one class of antibodies. Other
classes of antibodies include immunoglobulin M, D, A and E. One B
cell as it develops, can form all these different classes of antibodies
such that they will all have the same antigen-binding site but different
classes or tails of the 'Y' molecule. The class determines the mechanism
by which these antibodies will get rid of the antigen. These mechanisms
include triggering a group of lethal enzymes known as 'complement'
that makes holes in the membrane of the bacteria, thus destroying
it. Other antibodies bind to toxins and neutralize them by preventing
them from binding their target. These include antibodies that block
viruses from entering into cells (these are ideal for vaccinations
against viruses). There are other antibodies that coat (Process is
called as Opsonize) the bacteria and make them highly tempting to
scavenger cells, which will then engulf and eventually destroy them.
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