How Vaccines Work

How Vaccines Work.

The human immune system
is a complex network of cells and organs that evolved to fight off
infectious microbes. Much of the immune system’s work is carried out by
an army of various specialized cells, each type designed to fight
disease in a particular way. The invading microbes first run into the
vanguard of this army, which includes white blood cells called
macrophages (literally, “big eaters”). The macrophages engulf as many
of the microbes as they can.
Antigens Sound the Alarm

How do the macrophages
recognize the microbes? All cells and microbes wear a “uniform” made up
of molecules that cover their surfaces. Each human cell displays
unique marker molecules unique to you. Microbes display different
marker molecules unique to them. The macrophages and other cells of
your immune system use these markers to distinguish among the cells
that are part of your body, harmless bacteria that reside in your body,
and harmful invading microbes that need to be destroyed.
The molecules on a microbe that identify it as foreign and stimulate
the immune system to attack it are called “antigens.” Every microbe
carries its own unique set of antigens, which are central to creating
vaccines.

Macrophages digest most parts of the microbes but save the antigens and
carry them back to the lymph nodes, bean-sized organs scattered
throughout your body where immune system cells congregate. In these
nodes, macrophages sound the alarm by “regurgitating” the antigens,
displaying them on their surfaces so other cells, such as specialized
defensive white blood cells called lymphocytes, can recognize them.
Lymphocytes Take Over

There are two major kinds of
lymphocytes, T cells and B cells, and they do their own jobs in
fighting off infection. T cells function either offensively or
defensively. The offensive T cells don’t attack the microbe directly,
but they use chemical weapons to eliminate the human cells that have
already been infected. Because they have been “programmed” by their
exposure to the microbe’s antigen, these cytotoxic T cells, also called
killer T cells, can “sense” diseased cells that are harboring the
microbe. The killer T cells latch onto these cells and release
chemicals that destroy the infected cells and the microbes inside.

The defensive T cells, also called helper T cells, defend the body by
secreting chemical signals that direct the activity of other immune
system cells. Helper T cells assist in activating killer T cells, and
helper T cells also stimulate and work closely with B cells. The work
done by T cells is called the cellular or cell-mediated immune response.


B cells make and secrete extremely important molecular weapons called
antibodies. Antibodies usually work by first grabbing onto the
microbe’s antigen, and then sticking to and coating the microbe.
Antibodies and antigens fit together like pieces of a jigsaw puzzle—if
their shapes are compatible, they bind to each other.

Each antibody can usually fit with only one antigen. The immune system
keeps a supply of millions and possibly billions of different
antibodies on hand to be prepared for any foreign invader. It does this
by constantly creating millions of new B cells. About 50 million B
cells circulate in each teaspoonful of human blood, and almost every B
cell—through random genetic shuffling—produces a unique antibody that
it displays on its surface.
When these B cells come into contact with their matching microbial
antigen, they are stimulated to divide into many larger cells, called
plasma cells, which secrete mass quantities of antibodies to bind to the
microbe.

Antibodies in Action

The antibodies secreted by B
cells circulate throughout the human body and attack the microbes that
have not yet infected any cells but are lurking in the blood or the
spaces between cells. When antibodies gather on the surface of a
microbe, it becomes unable to function. Antibodies signal macrophages
and other defensive cells to come eat the microbe. Antibodies also work
with other defensive molecules that circulate in the blood, called
complement proteins, to destroy microbes.
The work of B cells is called the humoral immune response, or simply
the antibody response. The goal of most vaccines is to stimulate this
response. In fact, many infectious microbes can be defeated by
antibodies alone, without any help from killer T cells.

Clearing the Infection:

Memory Cells and Natural Immunity When
T cells and antibodies begin to eliminate the microbe faster than it
can reproduce, the immune system finally has the upper hand. Gradually,
the virus disappears from the body.
After the body eliminates the disease, some microbe-fighting B cells
and T cells are converted into memory cells. Memory B cells can quickly
divide into plasma cells and make more antibody if needed. Memory T
cells can divide and grow into a microbe-fighting army. If re-exposure
to the infectious microbe occurs, the immune system will quickly
recognize how to stop the infection.
How Vaccines Mimic Infection

Vaccines teach the
immune system by mimicking a natural infection. For example, the yellow
fever vaccine, first widely used in 1938, contains a weakened form of
the virus that doesn’t cause disease or reproduce very
well. Human macrophages can’t tell that the vaccine viruses are
weakened, so they engulf the viruses as if they were dangerous. In the
lymph nodes, the macrophages present yellow fever antigen to T cells
and B cells.
A response from yellow-fever-specific T cells is activated. B cells
secrete yellow fever antibodies. The weakened viruses in the vaccine are
quicky eliminated. The mock infection is cleared, and humans are left
with a supply of memory T and B cells for future protection against
yellow fever.


Source:-
http://www.niaid.nih.gov/topics/vaccines/understanding/pages/howwork.aspx

Hi friends,

This is the perfect blog for anyone who wants to know
about this topic. Human immune system is made up of interdependent cell
types which protect the body from parasitic fungal bacterial and viral
infections. It is in the lymph nodes where antigen is usually presented
to the immune system. Thanks for this information....

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