Medical Microbiology Chapter 14 – Immune Responses to Infectious Agents

Our Defense Systems  Our body has 3 layers/walls of defense against pathogens and toxins  1 st = barriers  2 nd = innate responses  3 rd = antigen-specific immune responses

Barriers  Barriers – Figure 14-1

Innate Responses  Innate Responses  Acute inflammation  Capillary dilation  Increased capillary permeability  Effects?  Exit of WBC to site of infection  Chemokines and chemotactic factors give a chemical trail to the infection  They lead to diapedesis  Figure 14-3

Innate Responses  Once at the site of infection, neutrophils and macrophages carry out phagocytosis  Figure 14-4  Complement  Chemotactic factors  Inflammation  Opsonins  Membrane attack complex  Interferon – “interferes” with viral infections (demo on board)

Innate Responses  Fever – can be stimulated by exogenous pyrogens (LPS) or endogenous pyrogens (cytokines)  Effect?  Assignment Assignment

Ag-specific Responses  Key features:  Must be produced after infection (usually takes 5-7 days)  Specificity - B-cells and T-cells are activated by antigens (Ag)  Memory – activated cells “remember” which Ag have entered the body

Ag-specific Responses  Activated B-cells produce Antibodies (Ab)  These are only effective against extracellular antigens  Activated cytotoxic (CD8) T-cells attack and destroy infected cells and cancer cells  These are only effective against intracellular antigens

Specific Immunity (cont.)  Activation of B-cells and T-cells:  Macrophages, dendritic cells, and other APCs carry out phagocytosis  They then process and present the Ag on MHC II  Helper (CD4) T-cells then come in contact with the APCs and become activated  Helper T-cells can then “help” B-cells and cytotoxic T-cells become activated  Demo on board

Specific Immunity (cont.)  B-cells come in contact with extracellular antigens (via membrane- bound Ab)  Protein Ag vs. Carbohydrate Ag  (for protein Ag) the Ag-Ab binding stimulates phagocytosis and Ag presentation  A helper T-cell then comes and gives the “2 nd signal” for activation  Once the cell is activated it proliferates and produces large amounts of Ab

B-cell Activation Antigen B-cell

B-cell Activation

Helper T-cell

B-cell Activation

Memory Cell

Antibody Mediated Immunity  Ab Functions – Figure from other text and hand-out

Antibody Mediated Immunity  Primary and secondary responses  Figure from other text

Cytotoxic T-cell Activation  Cytotoxic T-cell activation also requires helper T-cells  APCs present Ag on MHC II for helper T-cells and MHC I for cytotoxic T-cells  Once both cells recognize the Ag, the helper T- cells release cytokines  These allow the cytotoxic T-cells to become activated  Once the cell is activated it proliferates and “goes on patrol”  Demo on board

Immunizations  What does an immunization do?  A vaccine is made from antigens that are not harmful  Leads to memory and an secondary response to the real Ag  Figure from other text

Hypersensitivity  Hypersensitivity - immune reactions that occur in an exaggerated or inappropriate fashion  these immune responses cause damage to host tissues

Allergies  Allergies are basically an exaggerated immune response to a harmless substance  Hay fever is an allergy to plant pollen  the initial exposure to the Ag generates an immune response with large amounts of IgE produced  IgE can bind to mast cells and basophils and act as an Ag-receptor  during the next exposure to the Ag, the Ag will bind to the IgE on a mast cell or basophil

Allergies (cont.)  this results in the release of histamine and other chemicals  histamine triggers inflammation as well as runny nose, sneezing, etc.  How is hay fever usually treated?  Asthma is another form of allergy:  the same basic process applies, but the histamine released in the lungs causes the smooth muscles to contract  this closes off the air passages

Allergies (cont.)  Desensitization is a method to cure allergies  the person is given repeated doses of the Ag  How could this cure an allergy?  this stimulates the production of IgG and IgA  Neutralization of the Ag

Transplants & Autoimmunity  Transplant Rejection  Why would transplanted tissues be rejected?  Cytotoxic T-cells attack these cells just like they would a tumor cell or a virus-infected cell  Autoimmunity - the immune system reacting against normal body proteins  rheumatoid arthritis, others