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Parts of the Immune System
that Protect Against Infection
By Terry O. Harville, MD, PhD
LET’S REDEFINE THE term “immunity.” Immunity is
commonly referred to in a broad sense to represent the
processes of providing protection. In people, though, we
tend to think of it in narrow terms, frequently focusing
on the adaptive components — T and B lymphocytes.
Yet, in reality, protection from infections is a broader
process, and our immune system contains components
that we may not typically have considered as relevant for
immunity.
Skin and Mucosal Cells
Foremost is our skin, a critical barrier that keeps
pathogens out of the body. The outer layers of skin are
made from layers of flattened dead cells that are held
together, making it a more or less waterproof barrier,
impervious to minor trauma. There are numerous sweat
glands throughout the skin, and sweat contains chemicals
that may have antimicrobial properties.
A seemingly unlikely component of immune protection
is our urine. The urethra provides an opening for which
organisms may enter and cause bladder and, in worse
cases, kidney infections. Frequent urination can keep the
organisms “flushed” from the urinary tract, preventing
infections.
Another component is tears, which flush away organisms
from the eyes, as well as help defend against organisms
in the nose. In the early search for antimicrobial agents,
Sir Alexander Fleming, a Scottish bacteriologist, noted
that tears would cause bacteria to break apart (lyse). It
was found that tears contain digestive enzymes that are
capable of digesting the cell walls of bacteria. Saliva, yet
another component, is capable of doing the same. Saliva
contains many digestive enzymes capable of lysing the
cell walls of microorganisms. Therefore, tears and saliva
“bathe” our upper respiratory tract with antimicrobial
protection.
We also have several conduits into or through the
body, which may be pathways for pathogenic organisms
to invade our bodies. These include the ear canals,
Eustachian tubes, nose, tear ducts, mouth, lungs,
esophagus, stomach, intestines, anus, bladder and urethra.
Like the skin, these conduits are lined with cells, albeit
different from the skin, that are collectively known as the
mucosa. An interesting concept is that the open areas
bounded by these mucosal cell linings are actually “outside the body,” even though we may think of them as
“in” the body. In other words, the mucosal cells are like
the skin, but they line the otherwise open areas found
“inside” the body to protect them from the invasion of
organisms, and these cells connect directly to the outside
world via the mouth, anus, nose, etc. The mucosal cells
do not form the stratified layers of flattened cells like the
external skin, but they do have help from specialized cells
that secrete mucous. The mucous provides an additional
barrier over the cells, making it more difficult for organisms
to invade. In the ear canals, a waxy material is produced
for the same purpose. When considering the scale of size
of the milieu in which microorganisms live, the movement
of bacteria in water is comparable to humans swimming
through tar. And, mucous adds to this viscosity, further
impeding invader organisms.
Critical Protective Barriers
Clearly, the barrier provided by skin and mucosal cells
is a critical component of normal immune system protection. Further, the flow of fluids can help “flush” away
microorganisms, and many of the fluids have intrinsic
antimicrobial activities. In addition, the secretion of mucous
helps to further protect the mucosal cell layers. Although
not always recognized for their protective qualities, these
immune system components serve as our first line of
defense against infections.
Next time, we will explore more components of the
immune system.
TERRY HARVILLE, MD, PhD, is medical director of the Special
Immunology Laboratory at the University of Arkansas for
Medical Sciences, and a consultant for immunodeficiencies,
autoimmunities and transplantation.
June-July 2010
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