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IMMUNOLOGY IMMUNOLOGY 

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Infection and DiseasesInfection and Diseases

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InfectionInfection is the process by which the microorganismis the process by which the microorganism

enters into a relationship with the host.enters into a relationship with the host.

  Essential StepsEssential Steps

Entrance Multiplicaton Propagation

GIT

Genitourinary tract

Respiratory tract

Skin, mucous

membrane

Locally

Spread to target organ

Portal of exit

Mode of 

transmission

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Outcome of InfectionsOutcome of Infections

 Abortive Abortive

Subclinical or silentSubclinical or silent

ClinicalClinical

Outcome depends on:Outcome depends on:

11-- Microbial factors ( pathogenicity andMicrobial factors ( pathogenicity and

 virulence) virulence)

22-- Host resistance ( natural and acquired)Host resistance ( natural and acquired)

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Types of Microorganisms

Commensal or normal microbial flora

Opportunistic microorganisms

Pathogenic microorganisms

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Opportunistic MicroorganismsOpportunistic Microorganisms

(potential pathogens)(potential pathogens) They can cause disease under the following They can cause disease under the following

conditionsconditions::

11-- Host defence mechanisms are lowered.Host defence mechanisms are lowered.

22-- The natural habitat of the organism is The natural habitat of the organism is

changed.changed.

33-- Alteration in the host tissue occurs. Alteration in the host tissue occurs.

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Pathogenic MicroorganismsPathogenic Microorganisms

( true pathogens)( true pathogens) They are pathogens capable of infecting the tissues They are pathogens capable of infecting the tissues

of normal healthy subjects.of normal healthy subjects.

Properties of true pathogensProperties of true pathogens

Infectivity Virulence TransmissibilityInfectivity Virulence Transmissibility

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InfectivityInfectivity

PathogenPathogen portal of entry portal of entry   initiates infectioninitiates infection..

 The capacity to initiate infection depends on: The capacity to initiate infection depends on:

11-- Dose of pathogen.Dose of pathogen.

22-- Growth phase ( more infective in logGrowth phase ( more infective in log phase) phase)

33-- Virulence Virulence..

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 Virulence Virulence

It is the degree of pathogenicity of the organism.It is the degree of pathogenicity of the organism.

 Virulence Factors Virulence Factors

aa-- Adherence factors Adherence factors e.g. pili.e.g. pili.bb-- InvasivenessInvasiveness ( ability to invade tissue, multiply and spread( ability to invade tissue, multiply and spread

rapidly).rapidly).

cc-- Toxin production. Toxin production. ( endotoxins, exotoxins)( endotoxins, exotoxins)dd-- Extracellular enzymes.Extracellular enzymes. ( help to establish infection e.g.( help to establish infection e.g.

hyaluronidase, coagulase, fibrinolysin, collagenase).hyaluronidase, coagulase, fibrinolysin, collagenase).

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Characteristics of Exotoxins and EndotoxinsCharacteristics of Exotoxins and Endotoxins

ExotoxinsExotoxins

11-- Excreted by living cells.Excreted by living cells.

22-- Produced by Gram +ve andProduced by Gram +ve and

some Cramsome Cram  ± ± ve bacteria. ve bacteria.

33-- Polypeptides.Polypeptides.

44-- Relatively unstable.Relatively unstable.

55-- Highly immunogenic.Highly immunogenic.

EndotoxinsEndotoxins

11--Released upon bacterial death,Released upon bacterial death,

 part of Gram negative cell wall. part of Gram negative cell wall.

22-- Only found in GramOnly found in Gram  ± ± ve ve

bacteria.bacteria.

33-- LPS complex.LPS complex.

44-- Relatively stable.Relatively stable.

55-- Weakly immunogenic. Weakly immunogenic.

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Characteristics of Exotoxins and EndotoxinsCharacteristics of Exotoxins and Endotoxins

ExotoxinsExotoxins

66  ± ±Converted to toxoids.Converted to toxoids.

77-- Highly toxic.Highly toxic.

88-- Binds to specific receptorsBinds to specific receptors

on host cells, have selectiveon host cells, have selective

effect on tissues e.g.effect on tissues e.g.

leucocidins, haemolysins,leucocidins, haemolysins,necrotoxins.necrotoxins.

99-- Do not produce fever inDo not produce fever in

host.host.

EndotoxinsEndotoxins

66-- Not converted to toxoids.Not converted to toxoids.

77-- Less toxic.Less toxic.88-- No specific receptors on hostNo specific receptors on host

cells, produce nonspecificcells, produce nonspecific

effects as pyrexia, hypotention,effects as pyrexia, hypotention,

shock.shock.

99-- Usually produce fever byUsually produce fever by

release of ILrelease of IL--11 and otherand other

mediators.mediators.

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Sources of Infection to ManSources of Infection to Man

 A) Exogenous: A) Exogenous:

11-- Patients.Patients.

22-- Healthy carriers.Healthy carriers.

33-- Infected animals.Infected animals.

44-- Soil.Soil.

B) Endogenous:B) Endogenous:

11-- E. coli from gutE. coli from gut UTI.UTI.

22-- Staph aureus from noseStaph aureus from nose

skin boils.skin boils.

33-- Pneumococci fromPneumococci fromnasopharynxnasopharynx

bronchopneumonia.bronchopneumonia.

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C) Hospital acquired infections:C) Hospital acquired infections:

Infections may be acquired in the hospital :Infections may be acquired in the hospital :

11-- EndogenouslyEndogenously---- from patientfrom patient¶¶s own normal flora.s own normal flora.

22-- ExogenouslyExogenously--

---- from other peoplefrom other people (cross infection).(cross infection).-- from inanimate objects in the hospitalfrom inanimate objects in the hospital

environmentenvironment (environmental infection).(environmental infection).

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Modes of InfectionModes of Infection

11-- Respiratory infections.Respiratory infections.

22-- Skin, wound and burn infections.Skin, wound and burn infections.

33-- Venereal infections. Venereal infections.

44-- Alimentary tract infections ( faecal Alimentary tract infections ( faecal--oral route):oral route):

a) Fooda) Food--borne infection.borne infection.

b) Waterb) Water--borne infection.borne infection.

55-- Percutaneous transmission.Percutaneous transmission.

66-- Laboratory infection.Laboratory infection.