1. Chapter 43
The Immune System

2. Animal Immunity INNATE IMMUNITY Barrier defenses: Internal defenses:
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
Non-specific
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

3. I. Innate Immunity
Innate defenses include barrier defenses, phagocytosis, antimicrobial peptides.
Defenses are unique to vertebrates: the inflammatory response and natural killer cells.

4. A. Barrier Defenses
Skin and mucous membranes
Body fluids (saliva, mucus, and tears) are hostile to microbes.
Low pH of skin and the digestive system prevents growth

5. B. Cellular Innate Defenses
White blood cells = leukocytes engulf pathogens in the body (phagocytosis) then fuses with a lysosome to destroy the microbe.

6. Different types of phagocytic cells:
Neutrophils: engulf and destroy microbes.
Macrophages: part of the lymphatic system and found throughout body.
Eosinophils: discharge destructive enzymes.
Dendritic cells: stimulate development of acquired immunity.

7. Lymphatic System Interstitial fluid Adenoid Tonsil Blood capillary
nodes
Spleen
Tissue
cells
Lymphatic
vessel
Peyer’s patches
(small intestine)
Appendix
Figure 43.7 The human lymphatic system
Lymphatic
vessels
Lymph
node
Masses of
defensive cells

8. C. Inflammatory Responses
After injury, mast cells release histamine
increase local blood supply and more phagocytes and antimicrobial proteins to enter tissues.
Pus = a fluid (rich in white blood cells, dead microbes, and cell debris) accumulates at the site

9. 3. 1. 2. Major events in a local Inflammatory Response 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

10. D. Natural Killer Cells
All body cells (except RBC) have a class I MHC protein on their surface.
MHC = Major Histocompatibility Complex , part of the extracellular matrix.
Class II MHC protein molecules are found on specialized cells
Cancerous or infected cells no longer express this MHC protein; natural killer (NK) cells attack these damaged cells.

11. II. Acquired Immunity Lymphocytes recognize and respond to antigens
Lymphocytes from thymus (T cells) and from bone marrow (B cells)
Lymphocytes contribute to immunological memory,
Cytokines = secreted by macrophages and dendritic cells to recruit and activate lymphocytes.

12. B cells and T cells have receptor proteins that can bind to antigens (each cell binds to one antigen)

13. 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
B cell receptor
T cell receptor

14. A. Role of the MHC
In infected cells, MHC molecules bind and transport antigen fragments to the surface (antigen presentation)
T cell can then detect the antigen fragment

15. Antigen Presentation by an MHC molecule
Top view: binding surface
exposed to antigen receptors
Antigen
Class I MHC
molecule
Antigen
Figure Antigen presentation by an MHC molecule
Plasma
membrane of
infected cell

16. Class I MHC molecules are found on almost all nucleated cells of the body.
They display antigens to cytotoxic T cells.
Class II MHC molecules are found on specialized cells: macrophages, B cells, and activated T cells

17. Interaction of T cells with Antigen-Presenting Cells
Infected cell
Microbe
Antigen-
presenting
cell 1
Antigen
associates
with MHC
molecule
Antigen
fragment
Antigen
fragment 1 1
Class I MHC
molecule
Class II MHC
molecule 2 2
T cell
receptor
T cell
receptor 2
T cell
recognizes
combination
Figure The interaction of T cells with antigen-presenting cells
(a)
Cytotoxic T cell
(b)
Helper T cell

18. B. Amplifying Lymphocytes
The binding of a mature lymphocyte to an antigen induces the lymphocyte to divide rapidly (clonal selection)
Two types: short-lived effector cells (fight current battle) and long-lived memory cells… for future attacks by same pathogen.

19. Clonal Selection of B cells
Antigen molecules
B cells that
differ in
antigen
specificity
Antigen
receptor
Figure Clonal selection of B cells
Antibody
molecules
Clone of memory cells
Clone of plasma cells = effectors

20. First exposure to an antigen represents the primary immune response.
During this time, effector B cells = plasma cells are generated, and T cells are activated to their effector forms.
Secondary immune response = memory cells facilitate a faster, more efficient response.

21. Antibody concentration
Primary immune response
to antigen A produces
antibodies to A.
Secondary immune response to
antigen A produces antibodies to A.
Primary immune response to antigen
B produces antibodies to B.
104
103
Antibody concentration
(arbitrary units)
Antibodies
to A
102
Antibodies
to B
101
Figure The specificity of immunological memory
100 7 14 21 28 35 42 49 56
Exposure
to antigen A
Exposure to
antigens A and B
Time (days)

22. III. Acquired immunity
Humoral immune response = activation of B cells, production of antibodies.
Cell-mediated immune response = activation of cytotoxic T cells.
Helper T cells aid both responses.

23. 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.

24. A. Helper T Cells
Secrete cytokines that stimulate other lymphocytes.

25. The central role of helper T cells in humoral and cell-mediated immune responses
Antigen-
presenting
cell
Peptide antigen
Bacterium
Class II MHC molecule
CD4
TCR (T cell receptor)
Helper T cell
Cytokines
Positive Feedback … +
Humoral
immunity
= secretion of
antibodies by
plasma cells. +
Cell-mediated
immunity
= attack on
infected cells.
Figure The central role of helper T cells in humoral and cell-mediated immune responses + +
B cell
Cytotoxic T cell

26. B. Cytotoxic T Cells
Effector cells in cell-mediated immune response.
Activated cytotoxic T cell secretes proteins that destroy the infected target cell.

27. 3. lysis 1. 2. The killing action of cytotoxic T cells
Released cytotoxic T cell
Cytotoxic T cell
Perforin
Granzymes
CD8
TCR
Dying target cell
Class I MHC
molecule
Pore
Figure The killing action of cytotoxic T cells
For the Discovery Video Fighting Cancer, go to Animation and Video Files.
Target
cell
Peptide
antigen

28. C. B Cells
Humoral response = secretion of antibodies by B cells.

29. B cell activation in the humoral immune response
Antigen-presenting cell
Bacterium
Peptide
antigen
B cell
Class II MHC
molecule
Secreted
antibody
molecules +
Clone of plasma cells
TCR
CD4
Cytokines
Endoplasmic
reticulum of
plasma cell
Activated
helper T cell
Helper T cell
Clone of memory
B cells
Figure B cell activation in the humoral immune response
2 µm

30. D. Role of Antibodies
Neutralization = pathogen can no longer infect a host because it is bound to an antibody.
Opsonization = antibodies bound to antigens increase phagocytosis.

31. Antibody-mediated mechanisms of antigen disposal
Activation of complement system
and pore formation
Viral neutralization
Opsonization
Bacterium
Complement proteins
Virus
Formation of
membrane
attack complex
Flow of water
and ions
Macrophage
Pore
Figure Antibody-mediated mechanisms of antigen disposal
Foreign
cell

32. IV. Active Immunization
Active immunity = in response to an infection or vaccination
Body creates antibodies to fight off infection

33. V. Passive Immunity
Passive immunity = immediate, short-term protection.
Antibodies from mother to infant in breast milk or injected anitbodies

34. Passive immunization of an infant occurs during breast-feeding
Figure Passive immunization of an infant occurs during breast-feeding
For the Discovery Video Vaccines, go to Animation and Video Files.

35. VI. Immune Disruption

36. A. Allergies
Exaggerated (hypersensitive) responses to antigens (allergens)
Ex. hay fever, IgE antibodies produced after first exposure to an allergen attach to receptors on mast cells. Next time the allergen enters the body, it binds to mast cell–associated IgE molecules.
Mast cells release histamine and other mediators that cause vascular changes leading to typical allergy symptoms.

37. Mast cells, IgE, and the allergic response
Histamine
Allergen
Granule
Figure Mast cells, IgE, and the allergic response
Mast cell

38. B. Autoimmune Diseases
Immune system loses tolerance for self and turns against certain molecules of the body.
Ex. rheumatoid arthritis, insulin-dependent diabetes mellitus, and multiple sclerosis.

39. X-ray of a hand deformed by rheumatoid arthritis
Figure X-ray of a hand deformed by rheumatoid arthritis

40. C. Immunodeficiency Diseases
Inborn immunodeficiency results from hereditary or developmental defects that prevent proper functioning of innate, humoral, and/or cell-mediated defenses.
Acquired immunodeficiency results from exposure to chemical and biological agents.
Acquired immunodeficiency syndrome (AIDS) caused by HIV

41. VII. Immune System Evasion by Pathogens
Pathogens have evolved mechanisms to attack immune responses.
Through antigenic variation, some pathogens are able to prevent recognition.
Human viruses exchange genes with the viruses of domesticated animals.
Immune systems are unable to recognize the new viral strain. (Bird or Swine flu)

42. A. Latency
Some viruses may remain in a host in an inactive state
Herpes simplex viruses can be present in a human host without causing symptoms (cold sores, genital warts)

43. B. Attack on the Immune System: HIV
Human immunodeficiency virus (HIV) infects helper T cells.
The loss of helper T cells impairs both the humoral and cell-mediated immune responses and leads to AIDS.
HIV eludes the immune system because of antigenic variation and an ability to remain latent while integrated into host DNA.

44. Helper T cell concentration
The progress of an untreated HIV infection
Latency
AIDS
Relative antibody
concentration
800
Relative HIV
concentration
600
Helper T cell concentration
in blood (cells/mm3)
Helper T cell
concentration
400
Figure The progress of an untreated HIV infection
200 1 2 3 4 5 6 7 8 9 10
Years after untreated infection

45. You should now be able to:
Distinguish between innate and acquired immunity.
Name and describe four types of phagocytic cells.
Describe the inflammation response.

46. Distinguish between the following pairs of terms: antigens and antibodies; antigen and epitope; B lymphocytes and T lymphocytes; antibodies and B cell receptors; primary and secondary immune responses; humoral and cell-mediated response; active and passive immunity.
Explain how B lymphocytes and T lymphocytes recognize specific antigens.
Explain why the antigen receptors of lymphocytes are tested for self-reactivity.

47. Describe clonal selection and distinguish between effector cells and memory cells.
Describe the cellular basis for immunological memory.
Explain how a single antigen can provoke a robust humoral response.
Compare the processes of neutralization and opsonization.

48. Describe the role of MHC in the rejection of tissue transplants.
Describe an allergic reaction, including the roles of IgE, mast cells, and histamine.
Describe some of the mechanisms that pathogens have evolved to thwart the immune response of their hosts.
List strategies that can reduce the risk of HIV transmission.