To understand how vaccines work, it is helpful to first look at how the
body fights infections. When germs, such as bacteria and viruses infect
or invade the body, they multiply and damage body tissues causing
illness, which can be fatal. To ward off these germs, the body has an
army of cells called lymphocytes that it calls upon. The counter
offensive that the lymphocytes mount to eliminate the invading organisms
is called immune response. There are two types of lymphocytes, B cells
and T cells. Certain T cells, called cytotoxic T cells, destroy germs
that are inside cells, while B cells produce antibodies that bind to and
help kill the germs that are in tissues outside the cells. The first time the body encounters a particular germ, it mounts what
is called a primary immune response. This process takes a long time to
evolve because it consists of several steps. As the germ multiplies upon
entering the body, body cells called antigen presenting cells capture a
few of the germs and show these germs or parts of the germs (called
antigens) to certain T cells (helper T cells). The helper T cells that
have been shown the antigens then react by producing substances that
stimulate: some B cells to produce antibodies; some other T cells to
develop into cytotoxic T cells; and, some B and T cells to transform
into memory B and T cells, respectively. While the primary immune
response is building up, the individual gets and remains sick. Only
after enough antibodies and cytotoxic T cells have been produced, which
can take several days to weeks, will the infection be eliminated. An
unsuccessful primary immune response leads to death but a successful one
leads to recovery and immunity to the germ.
A successful immune response leaves behind a host of memory B and T
cells that can immediately react and mount a rapid (one to two days) and
vigorous response (called secondary immune response) every time the
same germ enters the body. Often, the germ gets eliminated without the
person even noticing that he/she has been infected that is why the
person is said to be immune to the infection.
Vaccines are designed to imitate an infection. This type of infection
however, does not cause illness. It simply stimulates the body to
undertake a primary immune response and produce memory B and T cells.
Vaccines vary in their composition. Some contain relatively harmless
substances (antigens) that consist of, or are similar to, whole or parts
of live or dead microorganisms; while others (called toxoids) consist
of bacterial toxins or poisons that have been modified to render them
nontoxic. In general, live vaccines confer life-long immunity after just
one vaccination, while dead microorganisms and toxoids require repeated
vaccinations and booster doses to maintain life-long protection.
Vaccines definitely work in preventing many potentially fatal
diseases, but are they safe? Like any medicine, vaccines are not 100
percent risk free, but scientific studies—the latest of which, published
in the July, 2014 issue of the medical journal Pediatrics, reviewed
20,000 scientific titles and 67 papers on vaccine safety—consistently
show that side effects from vaccines outside of soreness and redness at
the injection site are extremely rare. The British study that linked
autism to childhood vaccines that you have been told about has actually
been retracted after it was exposed to be an elaborate fraud. But there
is evidence that the meningococcal vaccine can lead anaphylaxis (severe
allergic reaction) while other studies link MMR vaccine to seizures.
