1. The Immune System

2. Overview Barriers = 1st line of defense against pathogens
Immune system recognizes foreign bodies
 responds with the production of leukocytes and proteins
2 kinds of defense:
innate immunity
acquired immunity

3. Pathogens (microorganisms and viruses)
INNATE IMMUNITY
Barrier defenses:
Skin
Mucous membranes
Secretions •
Recognition of traits
shared by broad ranges
of pathogens, using a
small set of receptors
Internal defenses:
Phagocytic cells
Antimicrobial proteins
Inflammatory response
Natural killer cells •
Rapid response
Figure 43.2 Overview of animal immunity
ACQUIRED IMMUNITY
Humoral response:
Antibodies defend against
infection in body fluids. •
Recognition of traits
specific to particular
pathogens, using a vast
array of receptors
Cell-mediated response:
Cytotoxic lymphocytes defend
against infection in body cells. •
Slower response

4. Innate immunity
present from birth
recognition and rapid response rely on shared traits of pathogens
nonspecific responses to pathogens
external barriers
internal cellular defense
chemical defenses
inflammation

5. Barriers
Skin - continuous keratinized stratified squamous layers of epithelium
Mucus traps and allows for the removal of microbes
Saliva, mucus, and tears are hostile to microbes (hydrolytic enzymes, high salinity)
low pH of skin and digestive system prevents microbial growth

6. Cellular Innate Defenses
EXTRACELLULAR
FLUID
Lipopolysaccharide
Flagellin
TLR4
TLR5
Helper
protein
TLR9
TLR3
WHITE
BLOOD
CELL
VESICLE
CpG DNA
ds RNA
Inflammatory
responses
Phagocytic leukocytes engulf pathogens in the body
Groups of pathogens are recognized by TLR, Toll-like receptors of leukocytes

7. Phagocytic Cells 4 types: Neutrophils engulf and destroy microbes
Macrophages (of lymphatic system) and are found throughout the body
Eosinophils discharge destructive enzymes
Dendritic cells stimulate acquired immunity

8. Chemical Defenses
Proteins and enzymes attack microbes directly or impede their reproduction
Interferon proteins:
innate defense against viruses
activate macrophages
~ 30 proteins form the complement system:
lysis of invading cells, trigger inflammation

9. Inflammatory Response
Following an injury, mast cells release histamine:
Dilates blood vessels
increase local blood supply
more phagocytes and antimicrobial proteins enter tissues
Pus, a fluid rich in white blood cells, dead microbes, and cell debris, accumulates at the site of inflammation

10. Inflammatory Response
Fever
100 – 102 Fahrenheit = beneficial
Slows growth and reproduction of microbes
denature their proteins

11. Pathogen Splinter Chemical signals Macrophage Fluid Mast cell
Capillary
Phagocytosis
Figure 43.8 Major events in a local inflammatory response
For the Cell Biology Video Chemotaxis of a Neutrophil, go to Animation and Video Files.
Red blood cells
Phagocytic cell

12. Natural Killer Cells
All cells in the body (except red blood cells) have a class 1 MHC protein on their surface (major histocompatibility complex)
Cancerous or infected cells lack this protein; natural killer (NK) cells attack these damaged cells

13. Innate Immune System Evasion by Pathogens
Some pathogens avoid destruction by modifying their surface to prevent recognition or by resisting breakdown following phagocytosis
Ex. Tuberculosis (TB);
kills more than a million people/year

14. Acquired immunity develops after exposure to foreign substances
specific responses
Antibody-mediated
Cell-mediated response
Slower

15. Lymphocyte-Dependent
lymphocytes recognize, remember, and respond to antigens, foreign molecules
Cytokines, secreted by macrophages and dendritic cells, recruit and activate lymphocytes
T cells Lymphocytes mature in the thymus above the heart
B cells mature in bone marrow

16. B and T-cell Specificity
Each lymphocyte is specialized to recognize ONE specific type of antigen (foreign body)
A single B cell or T cell has about 100,000 identical antigen receptors
B cells give rise to plasma cells, which secrete proteins called antibodies or immunoglobulins (Ig) specific to the antigen

17. Figure 43.9 Antigen receptors on lymphocytes
binding
site
Antigen-
binding site
Antigen-
binding
site V
Disulfide
bridge V V V
Variable
regions C V V C
Constant
regions C C C C
Light
chain
Transmembrane
region
Plasma
membrane
Heavy chains
 chain
 chain
Figure 43.9 Antigen receptors on lymphocytes
Disulfide bridge
B cell
Cytoplasm of B cell
Cytoplasm of T cell
T cell
(a) B cell receptor
(b) T cell receptor

18. Animation: Antibodies
The Role of Antibodies
Neutralization occurs when a pathogen can no longer infect a host because it is bound to an antibody
Opsonization occurs when antibodies bound to antigens increase phagocytosis
Antibodies together with proteins of the complement system generate a membrane attack complex and cell lysis
Animation: Antibodies

19. Antigen-binding sites
Fig
Antigen-
binding
sites
Epitopes
(antigenic
determinants)
Antigen-binding sites
Antibody A
Antigen V V
Antibody C V V C C C C
Figure Epitopes (antigenic determinants)
Antibody B

20. Animation: Role of B Cells
Primary Immune Response
The first exposure to an antigen
Slow response
B cells called plasma cells are generated,
T cells are activated
In secondary immune response, B-cell memory facilitate a faster, more efficient response
Animation: Role of B Cells

21. Two branches of Acquired immunity
Humoral (antibody-mediated) immune response
activation and clonal selection of B cells, resulting in production of secreted antibodies
Cell-mediated immune response
activation and clonal selection of cytotoxic T cells
Helper T cells aid both responses

22. Figure 43.16 An overview of the acquired immune response
Humoral (antibody-mediated) immune response
Cell-mediated immune response
Key
Antigen (1st exposure) +
Stimulates
Gives rise to
Engulfed by
Antigen-
presenting cell + + +
B cell
Helper T cell
Cytotoxic T cell + +
Memory
Helper T cells + + +
Figure An overview of the acquired immune response
Antigen (2nd exposure) +
Memory
Cytotoxic T cells
Active
Cytotoxic T cells
Plasma cells
Memory B cells
Secreted
antibodies
Defend against extracellular pathogens by binding to antigens,
thereby neutralizing pathogens or making them better targets
for phagocytes and complement proteins.
Defend against intracellular pathogens
and cancer by binding to and lysing the
infected cells or cancer cells.

23. Active Immunity
Active immunity develops naturally in response to an infection
It can also develop following vaccination
a nonpathogenic form of a microbe or part of a microbe is used to initiate a primary immune response and immunological memory