Medical Microbiology Chapter 14 – Immune Responses to Infectious Agents

Medical Medical MicrobiologyMicrobiology

Chapter 14 – Immune Responses to Chapter 14 – Immune Responses to Infectious AgentsInfectious Agents

Our Defense SystemsOur Defense Systems

Our body has 3 layers/walls of defense Our body has 3 layers/walls of defense against pathogens and toxinsagainst pathogens and toxins 11stst = barriers = barriers 22ndnd = innate responses = innate responses 33rdrd = antigen-specific immune responses = antigen-specific immune responses

BarriersBarriers

Barriers – Figure 14-1Barriers – Figure 14-1

Innate ResponsesInnate Responses

Innate Responses Innate Responses Acute inflammationAcute inflammation

Capillary dilationCapillary dilation Increased capillary permeabilityIncreased capillary permeability Effects?Effects?

Exit of WBC to site of infectionExit of WBC to site of infection Chemokines and chemotactic factors give a Chemokines and chemotactic factors give a

chemical trail to the infectionchemical trail to the infection They lead to diapedesisThey lead to diapedesis Figure 14-3Figure 14-3


Once at the site of infection, neutrophils and Once at the site of infection, neutrophils and macrophages carry out macrophages carry out phagocytosisphagocytosis

Figure 14-4Figure 14-4

ComplementComplement Chemotactic factorsChemotactic factors InflammationInflammation OpsoninsOpsonins Membrane attack complexMembrane attack complex

Interferon – “interferes” with viral infections Interferon – “interferes” with viral infections (demo on board)(demo on board)


Fever – can be stimulated by exogenous Fever – can be stimulated by exogenous pyrogens (LPS) or endogenous pyrogens pyrogens (LPS) or endogenous pyrogens (cytokines)(cytokines) Effect?Effect? AssignmentAssignment

Ag-specific ResponsesAg-specific Responses

Key features:Key features: Must be produced after infection (usually Must be produced after infection (usually

takes 5-7 days)takes 5-7 days) Specificity - Specificity - B-cells and T-cells are B-cells and T-cells are

activated by activated by antigens (Ag)antigens (Ag) Memory – Memory – activated cells “remember” activated cells “remember”

which Ag have entered the bodywhich Ag have entered the body

Ag-specific ResponsesAg-specific Responses

Activated B-cells produce Activated B-cells produce Antibodies (Ab)Antibodies (Ab) These are only effective against These are only effective against

extracellular antigensextracellular antigens Activated cytotoxic (CD8) T-cells Activated cytotoxic (CD8) T-cells

attack and destroy infected cells attack and destroy infected cells and cancer cellsand cancer cells These are only effective against These are only effective against

intracellular antigensintracellular antigens

Specific Immunity (cont.)Specific Immunity (cont.)

Activation of B-cells and T-cells:Activation of B-cells and T-cells: Macrophages, dendritic cells, and other APCs Macrophages, dendritic cells, and other APCs

carry out phagocytosiscarry out phagocytosis They then process and present the Ag on They then process and present the Ag on

MHC IIMHC II Helper (CD4) T-cells then come in contact with Helper (CD4) T-cells then come in contact with

the APCs and become activatedthe APCs and become activated Helper T-cells can then “help” B-cells and Helper T-cells can then “help” B-cells and

cytotoxic T-cells become activatedcytotoxic T-cells become activated Demo on boardDemo on board

Specific Immunity (cont.)Specific Immunity (cont.)

B-cells come in contact with B-cells come in contact with extracellular antigens (via membrane-extracellular antigens (via membrane-bound Ab)bound Ab) Protein Ag vs. Carbohydrate AgProtein Ag vs. Carbohydrate Ag (for protein Ag) the Ag-Ab binding (for protein Ag) the Ag-Ab binding

stimulates phagocytosis and Ag stimulates phagocytosis and Ag presentationpresentation

A helper T-cell then comes and gives the A helper T-cell then comes and gives the “2“2ndnd signal” signal” for activation for activation

Once the cell is activated it Once the cell is activated it proliferatesproliferates and and produces large amounts of Abproduces large amounts of Ab

B-cell ActivationB-cell Activation

Antigen

B-cell






Helper T-cell








Memory Cell

Antibody Mediated Antibody Mediated ImmunityImmunity

Ab Functions – Figure from other text Ab Functions – Figure from other text and hand-outand hand-out

Antibody Mediated Antibody Mediated ImmunityImmunity

Primary and secondary responsesPrimary and secondary responses Figure from other textFigure from other text

Cytotoxic T-cell ActivationCytotoxic T-cell Activation

Cytotoxic T-cell activation also requires Cytotoxic T-cell activation also requires helper T-cellshelper T-cells APCs present Ag on MHC II for helper T-cells and APCs present Ag on MHC II for helper T-cells and

MHC I for cytotoxic T-cellsMHC I for cytotoxic T-cells Once both cells recognize the Ag, the helper T-Once both cells recognize the Ag, the helper T-

cells release cytokinescells release cytokines These allow the cytotoxic T-cells to become activatedThese allow the cytotoxic T-cells to become activated

Once the cell is activated it Once the cell is activated it proliferatesproliferates and and “goes “goes on patrol”on patrol”

Demo on boardDemo on board

ImmunizationsImmunizations

What does an immunization do?What does an immunization do? A A vaccinevaccine is made from antigens that are is made from antigens that are

not harmfulnot harmful Leads to memory and an secondary response Leads to memory and an secondary response

to the real Agto the real Ag

Figure from other textFigure from other text

HypersensitivityHypersensitivity

HypersensitivityHypersensitivity - immune reactions - immune reactions that occur in an exaggerated or that occur in an exaggerated or inappropriate fashioninappropriate fashion these immune responses cause these immune responses cause

damage to host tissuesdamage to host tissues

AllergiesAllergies Allergies are basically an exaggerated immune Allergies are basically an exaggerated immune

response to a response to a harmless substanceharmless substance Hay fever is an allergy to plant pollenHay fever is an allergy to plant pollen

the initial exposure to the Ag generates an immune the initial exposure to the Ag generates an immune response with large amounts of response with large amounts of IgEIgE produced produced

IgE can bind to IgE can bind to mast cellsmast cells andand basophilsbasophils and act as and act as an Ag-receptoran Ag-receptor

during the next exposure to the Ag, the Ag will bind during the next exposure to the Ag, the Ag will bind to the IgE on a mast cell or basophilto the IgE on a mast cell or basophil

Allergies (cont.)Allergies (cont.)

this results in the release of this results in the release of histaminehistamine and and other chemicalsother chemicals

histamine triggers inflammation as well as histamine triggers inflammation as well as runny nose, sneezing, etc.runny nose, sneezing, etc.

How is hay fever usually treated?How is hay fever usually treated? Asthma is another form of allergy:Asthma is another form of allergy:

the same basic process applies, but the the same basic process applies, but the histamine released in the lungs causes the histamine released in the lungs causes the smooth muscles to contractsmooth muscles to contract

this closes off the air passagesthis closes off the air passages

Allergies (cont.)Allergies (cont.)

DesensitizationDesensitization is a method to cure allergies is a method to cure allergies the person is given repeated doses of the Agthe person is given repeated doses of the Ag

How could this cure an allergy?How could this cure an allergy? this stimulates the production of IgG and IgAthis stimulates the production of IgG and IgA

Neutralization of the AgNeutralization of the Ag

Transplants & Transplants & AutoimmunityAutoimmunity

Transplant RejectionTransplant Rejection Why would transplanted tissues be rejected?Why would transplanted tissues be rejected?

Cytotoxic T-cells attack these cells just like they Cytotoxic T-cells attack these cells just like they would a tumor cell or a virus-infected cellwould a tumor cell or a virus-infected cell

AutoimmunityAutoimmunity - the immune system reacting - the immune system reacting against normal body proteinsagainst normal body proteins rheumatoid arthritis, othersrheumatoid arthritis, others

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Medical Microbiology Chapter 14 – Immune Responses to Infectious Agents