The human body continuously faces attack from foreign
invaders that can cause infection and disease. These invaders range from
pathogens such as bacteria, fungi, parasites, and viruses, to non-living
toxins, chemicals, and drugs. Fortunately, though we are usually unaware of it,
our bodies are constantly in the process of defending against these microbial invaders.
This involves the use of several complex interacting
functions within the body. The human body have three lines of defence against
pathogens. The first two lines of defence are nonspecific; these are
ways in which the body attempts to destroy all types of substances that are
foreign to it. Nonspecific defence mechanisms include such things as mechanical
and physical barriers to invasion, chemical factors, microbial antagonism by
our indigenous microflora, phagocytic host cells, fever and the inflammatory
response.
The third line of defence, the immune response, is very
specific. Special proteins called antibodies are formed in response to the
presence of particular foreign substances.
First Line
of Defence
The first line of defence (or outside defence system) includes the physical and chemical barriers that are always ready and prepared to defend the body from pathogen invasion. These include the skin, tears, mucus, cilia, stomach acid, urine flow, microflora, bacteria and white blood cells.
If this defence is broken, the second line of defence within
the body is activated.
·
Intact Skin
The intact skin is acts as a waterproof physical or
mechanical barrier to pathogens unless it is damaged by injury. It also produces
normal secretions that destroy or inhibit the growth of microorganisms. These bactericidal
secretions include sweats (which are acidic) and an oily substance called sebum
(fatty acid). In addition normal flora already living on the skin prevents the
establishment of pathogens.
·
Mucus
Mucus produced at various mucous membranes at the openings to
the respiratory, digestive, urinary and reproductive systems entraps invaders.
·
Respiratory
System
Mechanisms such as the presence of hair in the nose, mucous
membrane and irregular chambers of the nose help to trap much of the debris in
inhaled air. Also the mucociliary covering of the respiratory tract sweep the
trapped dust and microbes toward the throat , where they are swallowed or
expelled by sneezing and coughing.
Also, lysozyme and other enzymes that destroy bacteria are
present in nasal secretions and tears. The microflora in the mouth and throat
also prevent pathogen inhabitation.
·
Tears
Tears contain lysozyme and other
enzymes that destroy bacteria.
·
Digestive
System
The digestive system is protected by the acidic content of the
stomach, digestive enzymes and the alkaline contents of the small intestines.
Also the mucous lining of the digestive tract trap and destroy invading
pathogens whereas the microflora in the intestines such as Escherichia coli, Entorococcus faecalis prevent the inhabitation of
pathogen. Peristalsis and the expulsion of faeces serve to remove pathogens
from the intestines.
·
Urinary
System and Reproductive System
The urinary tract of a healthy person is sterile and
therefore lacks the presence of any microorganism. Also the reproductive system
of the male and most of the reproductive organs in the female lack an indigenous
microflora. However microflora including opportunistic pathogens from the anus
and perineal skin may enter the vagina and invade the urethra. Microorganisms
are continually flushed from these areas by frequent urination and expulsion of
mucous secretions. Also the acidic urine and vaginal secretions also inhibit
microbial growth.
Second Line
of Defence
If the pathogen is able to get past the first line of
defence, for example, through a cut in your skin the second line of defence is
activated. This is also non-specific as it stops any type of microbe. The second
line of defence includes the nonspecific cellular and chemical responses to
microbial invasion. They include fever production, iron balance, phagocytosis
and clean up and repair of damaged areas.
·
Fever
Production
Fever is an abnormally high body temperature associated with
infection and is triggered by the release of prostaglandins. Fever may help to
combat infection by reducing the growth rate of pathogens and kill some
especially fastidious organisms. It may also increase metabolic activity of
body cells and activate heat shock proteins in order to strengthen the overall
immune response.
Fever may be beneficial, but beyond a tolerable limit it can
cause damage to a body's own enzymes.
·
Iron
Balance
Some pathogens use free iron in the body to produce poisonous
substances. In response to pathogen invasion, the body release substances that
stimulate iron storage in the liver and thus reduce the amount of free iron
available for the pathogen.
·
Phagocytosis
This is the process by which phagocytic white blood cells
(leukocytes) surround and engulf (ingest) foreign materials. Two important
phagocytes in the human body are macrophages and neutrophils. Phagocytes serve
as a ‘clean-up crew’ to rid the body of unwanted and often harmful substances
such as dead cells, microorganisms, debris and unused cellular secretions.
·
Inflammation
The body normally responds to any local injury, irritation or
microbial invasion through a series of events collectively called inflammation.
Vasodilation occurs shortly after tissue injury. This allows more blood to flow
to the site, bringing redness and heat. Vasodilation causes the endothelial
cells that line the capillaries to stretch and separate, causing increased
permeability. Plasma escapes from the blood vessels into the surrounding area,
causing the area to become oedematous (swollen) and subsequently pain.
The purpose of inflammation is to localise the infection,
prevent the spread of microbial invaders, neutralise toxins and aid in the
repair of damaged tissue. It is characterised by redness, heat, swelling, pain,
often pus formation and loss of function.
Third Line
of Defence
The immune response is the third line of defence against
pathogens. In this type, specific antibodies are produced to destroy specific
microorganisms. These antibodies are normally found in the blood plasma, lymph,
and other body secretions where they readily protect against the specific
pathogen that stimulated their production.
NB: Autoimmunity: This is the system of immune responses of
an organism against its own healthy cells and tissues.
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