Natural Resistance

9.5.1. Natural Resistance

It has been observed that the two cardinal aspects, namely : (a) physiological needs, and (b) meta- bolic 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 sta-

tus, level of nutrition potentiality, and certain intangible conditions viz., stress, mental agony, depres- sion etc.

Natural resistance essentially comprises of the following four vital and important aspects :

9.5.1.1. Species Resistance

In general, the fundamental physiologic characteristics of humans, namely : normal body tem- perature 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 cross- infect 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 environ-

ments, (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 gener- alized 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.

9.5.1.2. Racial Resistance

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 mem- bers die of progressive infection ultimately. It may be further expatiated by the following three glaring examples :

PHARMACEUTICAL MICROBIOLOGY

Examples :

(i) Incorporation of altogether ‘new pathogens’ e.g., tubercle bacillus, by the relatively resist- ant 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.

9.5.1.3. Individual Resistance

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 pro- file ; 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.

9.5.1.4. 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 ‘effec- tive barrier’, whereas hair follicles, dilatation of sweat glands, or abrasions do allow the gainful entry for the microbes into the human body.

* Who did not earlier developed a usual resistance to the organism. ** Immunodeficiencies : An inability to develop perfect immunity to pathogens.

IMMUNE SYSTEMS

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 con- tain a number of chemical substances which critically cause inhibition or destruction of microorganisms.

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

peptidoglycan,

(b) Several other hormones and enzymes are capable of producing distinct chemical, physi- ological, and mechanical effects that may ultimately cause minimization of susceptibil- ity 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

‘polymorphonuclear leukocytes’*.

(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.

* Polymorphonuclear Leukocytes : Leukocytes possessing a nucleus consisting of several parts or lobes connected by fine strands.

PHARMACEUTICAL MICROBIOLOGY

Mechanism : Transferrin (as well as Lactoferrin) critically undergoes ‘chelation’ with the bivalent ferrous iron [Fe 2+ ] available in the environment, thereby restricting profusely the availability of ferrous ion (i.e., an essential metal nutrient) to the particular invading microbes.