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Chapter 43
THE IMMUNE SYSTEM
Types of Immunity
Innate Immunity
• Non-specific
• All plants & animals
Adaptive Immunity
• Pathogen-specific
• Only in vertebrates
• Involves B and T cells
Figure 43.2
Pathogens
(such as bacteria,
fungi, and viruses)
INNATE IMMUNITY
(all animals)
• Recognition of traits shared
by broad ranges of
pathogens, using a small
set of receptors
• Rapid response
ADAPTIVE IMMUNITY
(vertebrates only)
• Recognition of traits
specific to particular
pathogens, using a vast
array of receptors
• Slower response
Barrier defenses:
Skin
Mucous membranes
Secretions
Internal defenses:
Phagocytic cells
Natural killer cells
Antimicrobial proteins
Inflammatory response
Humoral response:
Antibodies defend against
infection in body fluids.
Cell-mediated response:
Cytotoxic cells defend
against infection in body cells.
Antimicrobial Proteins:
Barrier Defenses:
• Interferons (inhibit viral
reproduction)
• Complement system (~30 proteins,
membrane attack complex)
•Skin
•Mucous membranes
•Lysozyme (tears,
saliva, mucus)
Innate
Immunity
(non-specific)
Natural Killer
Cells:
•Virus-infected and
cancer cells
Inflammatory Response:
• Mast cells release histamine
• Blood vessels dilate, increase
permeability (redness,
swelling)
• Deliver clotting agents,
phagocytic cells
• Fever
Phagocytic WBCs:
•Neutrophils
•Macrophage (“big eaters”)
•Eosinophils (parasite
destroyers)
•Dendritic
Phagocytosis
Inflammatory Response
Lymphatic System: involved in adaptive immunity
Adaptive Immunity
Lymphocytes (WBCs):
produced by stem cells in bone
marrow
• T cells: mature in thymus
– helper T, cytotoxic T
• B cells: mature in bone
marrow
– plasma cells make
antibodies
Adaptive Immunity
• 1. humoral immune response: antibodies help
neutralize or eliminate pathogens in the blood
and lymph
• 2. cell-mediated immune response specialized
T cells destroy affected host cells by apoptosis
• *BOTH are triggered by the helper T cells
• *BOTH make memory cells
• Antigen: substance that elicits lymphocyte
response
• Antibody (immunoglobulin – Ig): protein made by
B cell that binds to antigens
Figure 43.14
B cells that
differ in
antigen
specificity
Antigen
Antigen
receptor
Antibody
Memory cells
Plasma cells
Major Histocompatibility Complex (MHC)
• Proteins displayed on cell surface
• Responsible for tissue/organ rejection (“self” vs.
“non-self”)
• B and T cells bind to MHC molecule of antigen
presenting cell in adaptive response
Cytokines are chemical signals that attract B and T cells
Antigenpresenting
cell
Antigen fragment
Pathogen
Class II MHC molecule
Accessory protein
Antigen receptor
1
Helper T cell


Cytokines
Humoral
immunity
B cell

3
2

Cytotoxic T cell
Cellmediated
immunity
Antigenpresenting
cell
Cell-Mediated Immune
Response
(T Cells)
Humoral Immune Response
(antibodies)
Helper T
cell
B cell
Cytotoxic T
cell
Plasma
cell
tag for
destruction
Antibodies
Infected
cell
Identify and
destroy
Proliferation of B and T cells
• Once activated: B or T cell divides to produce
clones
• 1. effector cells that act immediately against the
antigen
• 2. memory cells make effector cells if the same
antigen is encountered again
Figure 43.18-3
Antigen-presenting
cell
Class II
MHC
molecule
Antigen
receptor
Pathogen
Antigen
fragment
B cell

Accessory
protein
Cytokines
Activated
helper T cell
Helper T cell
1
Memory B cells
2
Plasma cells
3
Secreted
antibodies
Cytotoxic T cells destroy infected cells with
Perforin (creates pores)
Cytotoxic T cell
Accessory
protein
Class I MHC
molecule
Infected
cell
1
Released
cytotoxic
T cell
Antigen
receptor
Perforin
Pore
Antigen
fragment
2
Dying
infected cell
Granzymes
3
Immunological Memory
• Primary immune response: 1st exposure to antigen
• Memory cells:
– Secondary immune response: repeat exposure 
faster, greater response
Figure 43.22
Histamine
IgE
Allergen
Granule
Mast cell
• Immunizations/vaccines: induce immune
memory to nonpathogenic microbe or toxin
• Passive immunity: via antibodies in breast milk
• Allergies: hypersensitive responses to
harmless antigens
• Autoimmune Diseases:
– Lupus, rheumatoid arthritis, Type I diabetes,
multiple sclerosis
• HIV: infect Helper T cells
– AIDS = severely weakened immune system
Helper T cell concentration
(in blood (cells/mm3)
Figure 43.25
Latency
AIDS
Relative anti-HIV antibody
concentration
800
Relative HIV
concentration
600
Helper T cell
concentration
400
200
0
0
1
3
7
8
2
4
5
6
Years after untreated infection
9
10