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:::Index>Profound Human World>Knowledge Acquisition>The Body's Defenses>The
Defense Mechanism
The Defense Mechanism
To protect the human body from the attack of outside pathogens, the body has erected
three defense lines. The first one is directly in contact with the outside world, like the
skin, respiratory tract, digestive tract and urinary tract. The skin is the first defense
line, and it is in direct contact with the outside world. When playing in the sand box
or swimming, all pathogens present in the environment is stopped by the skin’s total
protection.
Fig.1：Immune cells
IWe often say to be careful of what you eat, because the intestinal tract is also an
important line of defense. If rotten or unclean food is ingested by accident, the strong
gastric acids can kill all germs. Also, in the respiratory tract, nasal hairs in the nasal
cavity and cilia in the trachea can block foreign particle and pathogens. The urinary
tract also connects the body to the outside, and expels excess water and wastes. The
urethral mucosa’s particular pH value can impede pathogen entry to the body. Also,
with frequent urination, the microbes that accumulate at the opening of the urethra are
flushed out of the body, preventing pathogen entry. With the first defense line’s
protection, we can perform all kinds of activities without worry.
When the first line fails, as when for example we fall and break the skin, or when we
catch the flu and pathogens enter the respiratory duct, the body’s second line of
defense is activated. The body reacts with inflammatory responses: it could be a
localized inflammation, or it could be a system reaction like fever. White blood cells
begin to replicate in the body, in order to confront the outside attackers. White blood
cells with different specialties are assigned to contrast different types of invading
pathogens; neutrophils, macrophages and natural killer cells use phagocytosis against
pathogens. Cells and tissues attacked by pathogens secrete interferon, which forms a
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protective barrier that can reduce the pathogen’s attack speed. In hand-to-hand combat
with the pathogens, complements can attach themselves to the pathogens and
accelerate their demise.
When the fist two lines fail to stop the pathogens, then is time for the third and last
line to intervene. The human lymphatic system can produce two types of
lymphocytes, T-cells and the B-cells. These two cells have different functions and
actions. B cells are more like central control centers: they do not attack pathogens, but
remain in the lymphatic tissue to produce a large amount of antibodies to be sent
through the bloodstream to attack invaders, binding with surface antigens of infected
cells, blocking the pathogens’ action. T-cells are the foot soldiers; when infected cells
are targeted by antibodies, the antibodies bind to antigens to form an “antigen
complex”. At this time the pathogen has lost its virulence, and the T-cells can clear
these antigen complexes. After B-cells have produced antigens, they will remain in
the body for a while, so if there is an invasion of the same antigen, there are already
antibodies ready to attack them.
This is why, if we are infected with chicken pox once, we will never get chicken pox
again for the rest of our lives: this is because we have already acquired antibodies to
the chicken pox, and any new attack can be easily subdued, so there is no disease.
Through vaccination, we hope to use non-pathogenic viruses to stimulate antibody
production from the lymphocytes to prevent future viral attacks.
Reviewed by：Zhang, Yong-Ta professor
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