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Three Lines of Defense Against Infection: Skin, Mucous Membranes, and Immune System, Study notes of Immunology

ESCP Europe (École supérieure de commerce de Paris)Immunology

An overview of the body's defense mechanisms against pathogens, focusing on the three lines of defense: the physical barriers of the skin and mucous membranes, the innate immune response, and the adaptive immune response. It covers the functions and examples of each line of defense, as well as the role of specific immune cells and chemicals in protecting the body.

What you will learn

  • What are the three lines of defense against infection?
  • What are the functions of skin and mucous membranes as the first line of defense?
  • How does the immune system respond to pathogens?

Typology: Study notes

2021/2022

Uploaded on 09/27/2022

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Chapter 16:Chapter 16:
Nonspecific Host DefensesNonspecific Host Defenses
IntroductionIntroduction
ResistanceResistance: Ability to ward off disease.: Ability to ward off disease.
uuNonspecific Resistance:Nonspecific Resistance: Defenses that protect Defenses that protect
against against allall pathogens.pathogens.
uuSpecific Resistance:Specific Resistance: Protection against Protection against specificspecific
pathogens.pathogens.
SusceptibilitySusceptibility: Vulnerability or lack of : Vulnerability or lack of
resistance.resistance.
Protection Against Invading PathogensProtection Against Invading Pathogens
1. First Line of Defense1. First Line of Defense: : NonNon--specificspecific natural natural
barriers which barriers which restrictrestrictentryentry of pathogen. of pathogen.
ExamplesExamples: Skin and mucous membranes.: Skin and mucous membranes.
2. Second Line of Defense2. Second Line of Defense: Innate : Innate nonnon--specificspecific
immune defenses provide rapid local response to immune defenses provide rapid local response to
pathogen pathogen afterafterit has it has enteredenteredhost. host.
ExamplesExamples: Fever, phagocytes (macrophages and : Fever, phagocytes (macrophages and
neutrophilsneutrophils), ), inflammationinflammation, and interferon., and interferon.
3. Third line3. Third line of defense: of defense: AntigenAntigen--specificspecific immune immune
responses, specifically target and attack invaders responses, specifically target and attack invaders
that get past first two lines of defense. that get past first two lines of defense.
ExamplesExamples: Antibodies and lymphocytes.: Antibodies and lymphocytes.
Three Lines of Defense Against Infection
First Line of Defense:First Line of Defense:
Skin and Mucous MembranesSkin and Mucous Membranes
I. Mechanical DefensesI. Mechanical Defenses
1. Skin has two Layers:1. Skin has two Layers:
A. EpidermisA. Epidermis: Thin outer layer of : Thin outer layer of epithelialepithelial tissue.tissue.
Contains Contains Langerhans Langerhans cells, dead cells, and keratin cells, dead cells, and keratin
(waterproof).(waterproof).
B. DermisB. Dermis: Thick inner layer of : Thick inner layer of connectiveconnective tissue.tissue.
Infections are rare in intact skin. Exceptions:Infections are rare in intact skin. Exceptions:
FFHookworms can penetrate intact skinHookworms can penetrate intact skin
FFDermatophytesDermatophytes: “Skin loving” fungi: “Skin loving” fungi
Intact Skin is an Effective Barrier
Against Most Pathogens
pf3
pf4
pf5

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Chapter 16:Chapter 16:

Nonspecific Host DefensesNonspecific Host Defenses

IntroductionIntroduction

ResistanceResistance : Ability to ward off disease.: Ability to ward off disease.

uu Nonspecific Resistance:Nonspecific Resistance: Defenses that protectDefenses that protect againstagainst allall pathogens.pathogens. uu Specific Resistance:Specific Resistance: Protection againstProtection against specificspecific pathogens.pathogens.

SusceptibilitySusceptibility: Vulnerability or lack of: Vulnerability or lack of

resistance.resistance.

Protection Against Invading PathogensProtection Against Invading Pathogens

  1. First Line of Defense1. First Line of Defense:: NonNon--specificspecific naturalnatural barriers whichbarriers which restrictrestrict entryentry of pathogen.of pathogen. ExamplesExamples: Skin and mucous membranes.: Skin and mucous membranes.
  2. Second Line of Defense2. Second Line of Defense: Innate: Innate nonnon--specificspecific immune defenses provide rapid local response toimmune defenses provide rapid local response to pathogenpathogen afterafter it hasit has enteredentered host.host. ExamplesExamples: Fever, phagocytes (macrophages and: Fever, phagocytes (macrophages and neutrophilsneutrophils),), inflammationinflammation, and interferon., and interferon.
  3. Third line3. Third line of defense:of defense: AntigenAntigen--specificspecific immuneimmune responses, specifically target and attack invadersresponses, specifically target and attack invaders that get past first two lines of defense.that get past first two lines of defense. ExamplesExamples: Antibodies and lymphocytes.: Antibodies and lymphocytes.

Three Lines of Defense Against Infection

First Line of Defense:First Line of Defense:

Skin and Mucous MembranesSkin and Mucous Membranes

I. Mechanical DefensesI. Mechanical Defenses

  1. Skin has two Layers:1. Skin has two Layers: A. EpidermisA. Epidermis: Thin outer layer of: Thin outer layer of epithelialepithelial tissue.tissue. ContainsContains LangerhansLangerhans cells, dead cells, and keratincells, dead cells, and keratin (waterproof).(waterproof). B. DermisB. Dermis: Thick inner layer of: Thick inner layer of connectiveconnective tissue.tissue. Infections are rare in intact skin. Exceptions:Infections are rare in intact skin. Exceptions: FF Hookworms can penetrate intact skinHookworms can penetrate intact skin FF DermatophytesDermatophytes: “Skin loving” fungi: “Skin loving” fungi

Intact Skin is an Effective Barrier

Against Most Pathogens

I. Mechanical DefensesI. Mechanical Defenses

  1. Mucous Membranes2. Mucous Membranes: Line gastrointestinal,: Line gastrointestinal, genitourinary, and respiratory tracts.genitourinary, and respiratory tracts.

uu Two layers: Outer epithelial and inner connective layer.Two layers: Outer epithelial and inner connective layer.

uu Epithelial layer secretes mucus which maintains moistEpithelial layer secretes mucus which maintains moist surfaces.surfaces. uu Although they inhibit microbial entry, they offer lessAlthough they inhibit microbial entry, they offer less protection than skin.protection than skin. uu Several microorganisms are capable of penetratingSeveral microorganisms are capable of penetrating mucous membranes:mucous membranes: FF PapillomavirusPapillomavirus FF Treponema pallidumTreponema pallidum FF EE nteroinvasiventeroinvasive E.E. colicoli FF Entamoeba histolyticaEntamoeba histolytica

I. Mechanical DefensesI. Mechanical Defenses

3.3. LacrimalLacrimal apparatusapparatus: Continual washing and: Continual washing and blinking prevents microbes from settling on theblinking prevents microbes from settling on the eye surface.eye surface.

  1. Saliva4. Saliva: Washes microbes from teeth and mouth: Washes microbes from teeth and mouth mucous membranes.mucous membranes.
  2. Mucus5. Mucus: Thick secretion that traps many: Thick secretion that traps many microbes.microbes.
  3. Nose Hair6. Nose Hair: Coated with mucus filter dust, pollen,: Coated with mucus filter dust, pollen, and microbes.and microbes. 7.7. CiliaryCiliary EscalatorEscalator: Cilia on mucous membranes of: Cilia on mucous membranes of lower respiratory tract move upwards towardslower respiratory tract move upwards towards throat at 1throat at 1--3 cm/hour.3 cm/hour.

I. Mechanical DefensesI. Mechanical Defenses

  1. Coughing and sneezing8. Coughing and sneezing: Expel foreign objects.: Expel foreign objects.
  2. Epiglottis9. Epiglottis: Covers larynx during swallowing.: Covers larynx during swallowing.
  3. Urination10. Urination: Cleanses urethra.: Cleanses urethra.
  4. Vaginal Secretions11. Vaginal Secretions: Remove microbes from: Remove microbes from genital tract.genital tract.

Epiglottis Protects Respiratory System from

Infection During Swallowing

B. Chemical Defenses:B. Chemical Defenses:

uu SebumSebum: Oily substance produced by: Oily substance produced by sebaceous glands that forms a protective layersebaceous glands that forms a protective layer over skin. Contains unsaturated fatty acidsover skin. Contains unsaturated fatty acids which inhibit growth of certain pathogenicwhich inhibit growth of certain pathogenic bacteria and fungi.bacteria and fungi. uu pHpH: Low, skin pH usually between 3 and 5.: Low, skin pH usually between 3 and 5. Caused by lactic acid and fatty acids.Caused by lactic acid and fatty acids. uu PerspirationPerspiration: Produced by sweat glands.: Produced by sweat glands. ContainsContains lysozymelysozyme and acids.and acids. uu LysozymeLysozyme: Enzyme that breaks down gram: Enzyme that breaks down gram-- positive cell walls. Found in nasal secretions,positive cell walls. Found in nasal secretions, saliva, and tears.saliva, and tears.

B. Chemical Defenses (Continued)B. Chemical Defenses (Continued)

uu Gastric JuiceGastric Juice : Mixture of hydrochloric acid,: Mixture of hydrochloric acid, enzymes, and mucus. pH between 1.2 to 3enzymes, and mucus. pH between 1.2 to 3 kills many microbes and destroys most toxins.kills many microbes and destroys most toxins. Many enteric bacteria are protected by foodMany enteric bacteria are protected by food particles.particles. FF Helicobacter pyloriHelicobacter pylori neutralizes stomach acid andneutralizes stomach acid and can grow in the stomach, causing gastritis andcan grow in the stomach, causing gastritis and ulcers.ulcers. uu TransferrinsTransferrins: Iron: Iron--binding proteins in bloodbinding proteins in blood which inhibit bacterial growth by reducingwhich inhibit bacterial growth by reducing available iron.available iron.

Phagocytes are Attracted to Site of

Infection by Chemotaxis

Stages ofStages of PhagocytosisPhagocytosis (Continued)(Continued)

  1. Ingestion3. Ingestion: Plasma membrane of phagocytes: Plasma membrane of phagocytes extends projections (extends projections (pseudopodspseudopods) which engulf the) which engulf the microbe. Microbe is enclosed in a sac calledmicrobe. Microbe is enclosed in a sac called phagosomephagosome..
  2. Digestion4. Digestion: Inside the cell,: Inside the cell, phagosomephagosome fuses withfuses with lysosomelysosome to form ato form a phagolysosomephagolysosome.. LysosomalLysosomal enzymes kill most bacteria within 30enzymes kill most bacteria within 30 minutes and include:minutes and include: FF LysozymeLysozyme: Destroys cell wall: Destroys cell wall peptidoglycanpeptidoglycan FF Lipases and ProteasesLipases and Proteases FF RNAsesRNAses andand DNAsesDNAses After digestion,After digestion, residual bodyresidual body withwith undigestableundigestable material is discharged.material is discharged.

Process of Phagocytosis

InflammationInflammation

Triggered by tissue damage due to infection, heat,Triggered by tissue damage due to infection, heat, wound, etc.wound, etc. Four Major Symptoms of Inflammation:Four Major Symptoms of Inflammation: 1.1. RednessRedness

  1. Pain2. Pain
  2. Heat3. Heat
  3. Swelling4. Swelling May also observe:May also observe:
  4. Loss of function5. Loss of function

Functions of InflammationFunctions of Inflammation

1. Destroy and remove pathogens1. Destroy and remove pathogens

2. If destruction is not possible, to limit2. If destruction is not possible, to limit

effects by confining the pathogen and itseffects by confining the pathogen and its

products.products.

3. Repair and replace tissue damaged by3. Repair and replace tissue damaged by

pathogen and its products.pathogen and its products.

Stages of InflammationStages of Inflammation

1.1. VasodilationVasodilation: Increase in diameter of blood: Increase in diameter of blood vessels.vessels. Triggered by chemicals released by damaged cells:Triggered by chemicals released by damaged cells: histamine,histamine, kininskinins,, prostaglandinsprostaglandins, and, and leukotrienesleukotrienes..

  1. Phagocyte Migration and2. Phagocyte Migration and MarginationMargination:: MarginationMargination is the process in which phagocytesis the process in which phagocytes stick to lining of blood vessels.stick to lining of blood vessels. DiapedesisDiapedesis (Emigration)(Emigration): Phagocytes squeeze: Phagocytes squeeze between endothelial cells of blood vessels andbetween endothelial cells of blood vessels and enter surrounding tissue.enter surrounding tissue.

Process of Inflammation

Stages of Inflammation (Continued)Stages of Inflammation (Continued)

Phagocytes are attracted to site of infectionPhagocytes are attracted to site of infection throughthrough chemotaxischemotaxis.. Phagocytes destroy microbes, as well as dead andPhagocytes destroy microbes, as well as dead and damaged host cells.damaged host cells.

  1. Tissue Repair3. Tissue Repair: Dead and damaged cells are: Dead and damaged cells are replaced.replaced.

AntimicrobialAntimicrobial SubstancesSubstances::

I. Complement SystemI. Complement System: Large group of serum: Large group of serum proteins that participate in theproteins that participate in the lysislysis of foreignof foreign cells, inflammation, andcells, inflammation, and phagocytosisphagocytosis.. Two mechanisms of complement activation:Two mechanisms of complement activation:

  1. Classical Pathway1. Classical Pathway: Initiated by an immune: Initiated by an immune reaction ofreaction of antibodiesantibodies..
  2. Alternative Pathway2. Alternative Pathway: Initiated by direct: Initiated by direct interaction of complement proteins withinteraction of complement proteins with microbialmicrobial polysaccharidespolysaccharides.. Both pathways cleave a complement proteinBoth pathways cleave a complement protein called C3, which triggers a series of events.called C3, which triggers a series of events.

Classical Complement Pathway is Triggered by

Antibodies Binding to Foreign Cells

Both Classical and Alternative Complement

Pathways Trigger the Cleavage of C

Consequences of Complement Activation:Consequences of Complement Activation:

1.1. CytolysisCytolysis: Due to the formation of a: Due to the formation of a membranemembrane attackattack complexcomplex (MAC) which produces lesions in(MAC) which produces lesions in microbial membranes.microbial membranes. 2.2. InflammationInflammation: Complement components (C3a): Complement components (C3a) trigger the release of histamine, which increasestrigger the release of histamine, which increases vascular permeability.vascular permeability. 3.3. OpsonizationOpsonization: Complement components (C3b): Complement components (C3b) bind to microbial surface and promotebind to microbial surface and promote phagocytosisphagocytosis.. 4.4. Inactivation of ComplementInactivation of Complement: Regulatory proteins: Regulatory proteins limit damage to host cells that may be caused bylimit damage to host cells that may be caused by complement.complement.