1. Immune System

2. broad range of microbes
Acquired immunity allows the immune system to "remember" pathogens and prevent infection when that pathogen is encountered again
INNATE IMMUNITY
Rapid responses to a
broad range of microbes
ACQUIRED IMMUNITY
Slower responses to
specific microbes
External defenses
Internal defenses
Skin
Mucous membranes
Secretions
Phagocytic cells
Antimicrobial proteins
Inflammatory response
Natural killer cells
Humoral response
(antibodies)
Cell-mediated response
(cytotoxic
lymphocytes)
Invading
microbes
(pathogens)

4. Phagocytic leukocyte
Pathogen

5. Bacteria are surrounded Microbes
MACROPHAGE
Vacuole
Lysosome
containing
enzymes
Bacteria are engulfed
Vacuole forms around bacteria
The vacuole fuses with a lysosome
Enzymes break down the bacteria
Bacteria fragments exit the cell

6. The most numerous phagocytic leukocyte in the human body are neutrophils
When tissues are infected, they send out chemical signals that attract neutrophils

8. Two other types of phagocytic leukocytes are eosinophils and dendritic cells
Eosinophils engulf parasites
Dendritic cells are important for the development of acquired immunity

9. Fluid, antimicrobial proteins, and clotting elements move
Pathogen
Pin
Macrophage
Chemical signals
Capillary
Phagocytic cells
Red blood cell
Blood
clotting
elements
Blood clot
Phagocytosis
Fluid, antimicrobial proteins,
and clotting elements move
from the blood to the site.
Clotting begins. 2
Chemical signals released
by activated macrophages
and mast cells at the injury
site cause nearby capillaries
to widen and become more
permeable. 1
Chemokines released by various
kinds of cells attract more
phagocytic cells from the blood
to the injury site. 3
Neutrophils and macrophages
phagocytose pathogens and
cell debris at the site, and the
tissue heals. 4

11. Blood, lymph, and lymphoid tissues (lymph nodes, spleen, and others)
Bone marrow
Lymphoid
stem cell
Thymus
B cell
T cell
Blood, lymph, and lymphoid tissues
(lymph nodes, spleen, and others)

12. An antigen may have several different epitopes, all of which are capable of being recognized by a different receptor
Antigen-
binding
sites
Antibody A
Antigen
Antibody B
Antibody C
Epitopes
(antigenic
determinants)

13. Antigen-binding site Antigen-binding site Variable regions Constant
Plasma
membrane
B cell
Cytoplasm of B cell

14. Antigen- Binding site Variable regions Plasma membrane T cell Constant

15. Infected cell
A fragment of
foreign protein
(antigen) inside the
cell associates with
an MHC molecule
and is transported
to the cell surface. 1
Antigen
fragment
Class I MHC
molecule
The combination of
MHC molecule and
antigen is recognized
by a T cell, alerting it
to the infection.
T cell
receptor
(a) Killer T cell

16. Microbe
Antigen-
presenting
cell
A fragment of
foreign protein
(antigen) inside the
cell associates with
an MHC molecule
and is transported
to the cell surface.
Antigen
fragment
Class II MHC
molecule
The combination of
MHC molecule and
antigen is recognized
by a T cell, alerting it
to the infection.
T cell
receptor
Helper T cell

17. Antibody concentration
(arbitrary units)
104
103
102
101
100 7 14 21 28 35 42 49 56
Time (days)
Antibodies
to A
to B
Primary
response to
antigen A
produces anti-
bodies to A 2
Day 1: First
exposure to 1
Day 28:
Second exposure
to antigen A; first
antigen B 3
Secondary response to anti-
gen A produces antibodies
to A; primary response to anti-
gen B produces antibodies to B 4

18. Humoral immune response Cell-mediated immune response
First exposure to antigen
Intact antigens
Antigens engulfed and displayed by dendritic cells
Antigens displayed by infected cells
Activate
Gives rise to
B cell
Helper T cell
Cytotoxic T cell
Plasma cells
Memory B cells
Active and memory helper T cells
Memory cytotoxic T cells
Active cytotoxic T cells
Secrete antibodies that defend against pathogens and toxins in extracellular fluid
Defend against infected cells, cancer cells, and transplanted tissues
Secreted
cytokines
activate

19. A dendritic cell engulfs a bacterium
A dendritic cell engulfs a bacterium. The antigen is displayed on the dendritic cell’s class II MHC surface molecule. This is antigen presentation. The helper T cell binds to the antigen and is activated. 1
Cytotoxic T cell
Dendritic
cell
Peptide antigen
Helper T cell
Class II MHC
molecule
Attack on
infected cells
Bacterium 2 3 1
CD4
Secretion of
antibodies by
plasma cells
Dendritic
cell
Cytokines
B cell
The helper T cell makes multiple copies of itself (clonal selection) and releases chemical messengers. 2 3
The chemical messengers target killer (cytotoxic) T cells and B cellls, activating them

20. Killer T cells recognize infected cells and bind to them.
The activated killer T cell punches holes in the infected cell’s membrane and injects it with a poison. 1 2 3
The infected cell dies.
Killer T cells recognize infected cells and bind to them.
Cytotoxic T cell
Released
cytotoxic
T cell
Cancer
cell 1
CD8 3
Class I MHC
molecule 2
Target
cell
Peptide
antigen
Cytotoxic
T cell