Understanding the Immune System: Defenses Against Microbial Infections

1. ImmunologyThe Body’s Defenses Chapter 33

2. How Microbes Cause Disease • pathogen • any disease-causing organism • antigens (ag) • any substance that triggers the immune system to respond • infections can be superficial or systemic, or one then the other • bacteria • adherence (with adhesions orfimbriae) • colonization  invasiveness • toxins: damage to cells/tissues  damage to host • enzymes: increase virulence, degrade cells/tissues, cause/dissolve clots • capsule: helps resist phagocytosis • viruses • replication inside host cells (lytic and lysogenic cycles) • fungi • usually cause superficial infections through enzymes (e.g., keratinase) • allergic reactions • toxins • protists (protozoa) • ingest host cells and fluids • invade rbc’s • algae • neurotoxins • disease transmission can be direct or indirect

3. Host Defense • nonspecific defenses • very general (broad spectrum)  work on any antigen • not as strong as specific defenses • first line • anatomical barriers • intact skin and secretions of skin • saliva and mucous • coughing and sneezing reflexes • normal flora • bacteria normally living in body  compete with pathogens • slow down growth of pathogen or spread of antigen • second line • phagocytic leukocytes (wbc's) • neutrophils: release some destructive chemicals • eosinophils: defense against larger parasites • monocytes: immature macrophages • macrophages: act as antigen-presenting cells (APC's) • dendritic cells: act as antigen-presenting cells (APC's)

4. Fig. 33.5 A macrophage engulfing multiple bacteria • other nonspecific leukocytes (notphagocytic) • basophils: release histamine and heparin • natural killer (NK) cells: destroy viral-infected and tumor cells

5. antimicrobial substances • tears (lysozyme) • transferrins • iron-binding proteins in blood • reduce available Fe for pathogen • molecular defenses • interferon • anti-viral protein produced by infected cells • some antitoxins • neutralize toxins • complement system • 20+ proteins in blood • many functions • inflammation and fever • confine infection • raise temp. above pathogen’s normal range Fig. 33.6 Action of the complement system against bacteria

6. Fig. 33.3 A summary of nonspecific defenses

7. specific defenses (third line of defense) • respond only to one antigen at a time (narrow spectrum) • stronger than nonspecific defenses • specific leukocytes (lymphocytes) • T-cells • B-cells  plasma cells • antibodies (immunoglobulins; ab) • proteins that bind specifically (lock-and-key) to antigens (ag) • some antitoxins • Specific Immunity • kinds of immunity • innate: genetics or 1st and 2nd line only • acquired: some way other than genetics; involves specific defenses • active: body makes it’s own ab’s • naturally acquired active: by having a disease • artificially acquired active: through vaccine • weakened or dead form of antigen  causes immune response • passive: body obtains ab’s through external source • naturally acquired passive: across placenta or in breast milk • artificially acquired passive: through immune serum • antigens (ag) • usually parts of pathogens • foreign proteins or carbs of certain size • haptens and allergens

8. cells and tissues involved in specific immunity • specific leukocytes (lymphocytes) • B-cells produceab • become plasma cells secreteab • memory cells  protect if invaded by same pathogen again • T-cells • cytotoxic (Tc) • destroy viral-infected, tumor, or foreign cells • helper (Th) • activate Tc, B-cells, and other immune cells • suppressor (Ts) • turn immune system off after infection • memory (Tm) • become Tc or Th to protect against same antigen • circulatory and lymphatic systems • transport immune substances to site of infection • leukocytes concentrate and mature in lymph nodes • immune surveillance • lymph nodes, Tc, macrophages, NK cells • four general properties • recognition, specificity, heterogeneity, memory

9. Fig. 33.14 Action of cytotoxic T-cells

10. Cytotoxic T-cells attacking and destroying a cancer cell (target cell)

11. Dual Nature of Immune System • humoral immunity (antibody-mediated) • consists of: • B-cells, plasma cells, memory cells • Ab circulating in blood • defends against extracellular pathogens and free ag • properties and structure of Ab’s • heavy vs. light chains • constant vs. variable regions • classes of Ab’s • IgG • IgM • IgA • IgE • IgD Fig. 33.11 Structure of an antibody

12. The five classes of antibodies

13. ab’s work by neutralizing an ag • neutralization  renders ag ineffective • ways of accomplishing this: • coat ag to prevent adherence • enhance phagocytosis • coat surface of ag • opsonization • clump many ag’s together • agglutination • precipitate soluble antigen • act as antitoxins • trigger inflammation and fever • activate complement system

14. T-cell influence • Th cells bring ag to B-cells  activate B-cells ab produced The action and work of antibodies

15. cell-mediated immunity • consists of: • direct action of T-cells (esp., Th and Tc) • nonspecific leukocytes • phagocyticwbc’s, basophils, NK cells • defends against intracellular pathogens and cancer • process: • some macrophages act as antigen-presenting cells (APC's) • bring ag to Th cells Th cells activated • Th cells bring ag to B-cells and Tccells  B-cells and Tc cells activated • major histocompatibility complex (MHC) • protein on all of an individual’s cells that identifies “self” tissue • immune cells communicate with each other through various chemicals

16. Fig. 33.7 A summary of specific defenses

17. Sequence of Events Occurring During a Typical, First-Time Infection • pathogen (ag) invades and damages body  nonspecific defense activate (2nd line)  macrophage phagocytizes a pathogen  macrophage displays ag on its surface  macrophage presents (APC) ag to a Th cell  Th cell brings ag to B-cells and activates other T-cells (esp., Tc cells)  B- cells produce ab in response to ag some B-cells become plasma cells  plasma cells release abab’s and various T-cells begin attacking ag some B-cells and T-cells (Th, Tc) become memory cells near end of infection  Ts cells turn immune system off

18. An overview of the immune response

19. Immunologic Memory • immune system remembers what it has been exposed to previously • responds very quickly and efficiently to such ag’s • memory B-cells  become plasma cells  release large amts. of ab • memory T-cells (Tm)  become Th and Tc cells  quickly attack ag • most often, symptoms do not even occur • primary vs. secondary response • Factors That May Modify the Immune Response • compromised host • genetics • age • nutrition • effect of injury • environment • stress Fig. 33.9 Primary and secondary response