In a broader sense the ensuing interaction existing between a host (human body) and a
microorganism designates an excellent unique dynamic phenomenon whereby each and every protagonist
critically serves to maximize its overall survival. It has been duly observed that in certain typical instances,
after a specific microbe gains its entry or comes in contact with a host, a distinct positive mutually
beneficial relationship takes place which ultimately becomes integral to the final health of the host. In
this manner, the microorganisms turn out to be the normal microbiota*. However, in other such cases,
the particular microorganism causes, induces or produces apparent devastating and deleterious overall
effects upon the host ; and, therefore, may finally even cause death of the host via a dreadful ailment.
Interestingly, the prevailing environment of a ‘host’ is heavily surrounded with microorganisms,
and there lies an ample scope and opportunity to come in their contact every moment of the day.
Nevertheless, quite a few of these microbes are pathogenic in nature (i.e., cause disease). Surprisingly,
these pathogens are at times duly guarded and prevented from producing a disease due to the inherent
competition offered by the normal microbiota. In reality, the invading pathogens are squarely kept
away from the host by the ‘normal microbiota’ by using nutrients, resources, space, and may even yield
such chemical substances which would repel them ultimately.
In addition to the above stated glaring scientific fact and evidences these ‘normal microbiota’
grossly prevent colonization of pathogens to a great extent ; and, thereby, most probably checking the
disease (to the host) via ‘bacterial interference’.
Example : An excellent typical example is stated as under :
Lactobacilli – present strategically in the female genital tract (FGT) usually maintain a low pH
(acidic), and thereby exclusively afford the colonization by the pathogenic microbes. Besides, the
corynebacteria located critically upon the skin surface give rise to the formation of ‘fatty acids’ which
ultimately inhibit the phenomenon of colonization by the pathogenic organisms.
Note : It is an excellent example of ‘amensalism’. (i.e., symbiosis wherein one population (or individual)
gets affected adversely and the other is unaffected).
Interestingly, the ‘normal microbiota’ usually give rise to protection confined to a certain degree
from the invading pathogens ; however, they may themselves turn into pathogenic in character and
cause disease under certain particular circumstances. Thus, these ‘converted pathogens’ are invariably
known as ‘opportunistic microorganisms’** or pathogens.
Based on the above statement of facts and critical observations one may conclude that on one
hand pathogen makes use of all the opportune moments available at its disposal to cause and induct
infection, the host’s body possesses a plethora of ‘defense mechanisms’ to encounter the infection. In
fact, the observed intricacies prevailed upon by the host-pathogen relationship are not only numerous
but also quite divergent in nature, which may be classified under the following three heads, such as :
(a) Natural Resistance,
(b) Internal Defense Mechanisms, and
(c) Nonspecific Defense Mechanisms.
The aforesaid three categories shall now be discussed separately in the sections that follows :
It has been observed that the two cardinal aspects, namely : (a) physiological needs, and (b) metabolic
requirements, of a pathogen are an absolute necessity in establishing precisely the extent vis-a-vis
the range of potentially susceptible hosts. However, the naturally resistant hosts exert their action in
two variant modes, such as :
miserably fail to cater for certain urgently required environmental factors by the microbes
for their usual growth, and
essentially possess defense mechanisms to resist infection considerably.
Besides, there are some other factors pertaining to the host’s general health, socioeconomic status,
level of nutrition potentiality, and certain intangible conditions viz., stress, mental agony, depression
Natural resistance essentially comprises of the following four vital and important aspects :
In general, the fundamental physiologic characteristics of humans, namely : normal body temperature
may give a positive clue whether or not a specific bacterium can be pathogenic in nature.
Likewise, in host-specific e.g., human and bovine species, the tubercle bacillus is found to crossinfect
both humans and cattle having almost an identifical body temperature.
Salient Features : The salient features of species resistance are as given under :
(1) inability of a bacterium to induct disease in the resistant species under the natural environments,
(2) critical production in the specific resistant species of either a localized or a short-period
infection caused solely due to an experimental inoculation vis-a-vis a progressive or generalized
ailment in naturally susceptible species, and
(3) introduction of experimental disease particularly in the resistant species exclusively caused
by massive doses of the microbes, usually in two different ways :
(a) under unnatural parameters, and
(b) by an unnatural route.
Exhaustive and intensive studies have amply proved that the very presence of a pathogen in the
isolated races give rise to a gradual selection for resistant members, because the susceptible members
die of progressive infection ultimately. It may be further expatiated by the following three glaring
(i) Incorporation of altogether ‘new pathogens’ e.g., tubercle bacillus, by the relatively resistant
Europeans into an isolated American Indians population*, finally caused epidemics
that almost destroyed a major proportion of the ensuing population.
(ii) African Blacks (Negros) invariably demonstrate a relatively high resistance to the tropical
diseases, namely : malaria, yellow fever, and
(iii) Orientals do exhibit a much reduced susceptibility to syphilis.
It may be critically observed that there are certain individuals who apparently experience fewer
or less severe infections in comparison to other subjects, irrespective of the fact that :
both of them essentially possess the same racial background, and
do have the same opportunity for ultimate exposure.
Causation : Individual resistance of this nature and kind is perhaps on account of :
natural in-built resistance factor, and
adaptive resistance factor.
Age Factor – is equally important, for instance :
aged people are more prone to such ailments as : Pneumonia – most probably due to a
possible decline of the ‘immune functions’ with advancement in growing age.
children i.e., very young individuals are apparently more susceptible to such ‘children’s
disease’ as : Chicken-pox, measles–just prior to their having acquired enough in-built
resistance/immunity that essentially follows both inapparent and overt contracted infections.
Genetic Factor – Immunodeficiencies** found in some, individuals are caused solely due to
‘genetic defects’, that largely enhance the probability and susceptibility to disease.
Other Factors – include malnutrition, personal hygiene, and an individual’s attitude to sex profile
; hazards and nature of work-environment ; incidence of contacts with infected individuals, and an
individual’s hormonal vis-a-vis endocrine balance – they all do affect the overall frequency as well as
selectivity of some critical ailments.
External Defense Mechanisms
In fact, the external defense mechanisms do represent another cardinal and prominent factor in
natural resistance ; however, they essentially involve the chemical barriers as well. Besides, two other
predominant factors viz., (a) mechanical barriers, and (b) host secretions, essentially make up the
body’s First-Line of Defense Mechanism against the invading microorganisms.
Mechanical Barriers – actually comprise of such materials as : intact (unbroken) skin and
mucous membranes that are practically incapable of getting across to the infectious agents. However,
the said two mechanical barriers viz., intact skin and mucous membranes do afford a substantial ‘effective
barrier’, whereas hair follicles, dilatation of sweat glands, or abrasions do allow the gainful
entry for the microbes into the human body.
Examples : Various typical examples are as given under :
(1) Large segment of microbes are duly inhibited by such agents as :
low pH (acidity),
lactic acid present in sweat, and
fatty acids present in sweat.
(2) Mucous secretions caused by respiratory tract (RT), digestive tract (DT), urogenital
tract (UT) plus other such tissues do form an integral protective covering of the respective
mucous membranes thereby withholding and collecting several microorganisms until they
may be either disposed of effectively or lose their infectivity adequately.
(3) Chemical Substances – Besides, the ensuing mechanical action caused by mucous, saliva,
and tears in the critical removal of microorganisms, quite a few of these secretions do contain
a number of chemical substances which critically cause inhibition or destruction of
Examples : A few typical examples are as stated under :
(a) Lysozyme – an enzyme invariably observed in several body fluids and secretions viz.,
blood, plasma, urine, saliva, cerebrospinal fluid, sweat, tears etc., that predominantly do
exert an effective antimicrobial action on account of its inherent ability to lyse some
particular Gram positive microbs by specifically affording the hydrolysis of
(b) Several other hormones and enzymes are capable of producing distinct chemical, physiological,
and mechanical effects that may ultimately cause minimization of susceptibility
to reduction, and
(c) The prevailing inherent acidity or alkalinity of certain ‘body fluids’ possess an apparent
deleterious effect upon several microbes, and helps to check and prevent the potential
pathogens for gaining an easy access to the deeper tissues present in the body.
(d) Lactoferrin-Lactoferrin is an iron-containing red-coloured protein found in milk (viz.,
human and bovine) that essentially possesses known antibacterial characteristic features.
It is also found in a plethora of body-secretions that specifically and profusely bathe the
human mucosal surfaces, namely :
• bronchial mucous ; • seminal fluids ;
• hepatic bile ; • saliva ;
• nasal discharges ; • tears ; and
• pancreatic juice ; • urine.
Lactoferrin forms a vital and important constituent of the highly particular granules of the
(5) Transferrin : It represents the serum counterpart of lactoferrin. In fact, both these typical
proteins essentially possess high molecular weights ~ 78,000 daltons, besides having several
metal-binding critical sites.