1. Chapter 43
The Immune System

2. Concept 43.2: In acquired immunity, lymphocytes provide specific defenses against infection
Acquired immunity is the body’s second major kind of defense
An antigen is a foreign molecule that is recognized by lymphocytes and elicits a response from them
A lymphocyte recognizes and binds to a small portion of the antigen called an epitope

3. Acquired Immunity: occurs when the body adapts to specific infections; it’s like the body learns about the “invader” and kills it as effectively as possible.

4. (antigenic determinants) Recognition sites for an antibody Antibody A
LE 43-7
Antigen-
binding
sites
Epitopes
(antigenic determinants)
Recognition sites for an
antibody
Antibody A
Antigen
Antibody B
Antibody C

5. Antigen Recognition by Lymphocytes
Two main types of lymphocytes circulate in the blood of vertebrates: B lymphocytes (B cells) and T lymphocytes (T cells)
A single B cell or T cell has about 100,000 identical antigen receptors
All antigen receptors on a single cell recognize the same epitope

6. B Cell Receptors for Antigens
B cell receptors bind to specific, intact antigens
Secreted antibodies, or immunoglobulins, are structurally similar to B cell receptors but lack transmembrane regions that anchor receptors in the plasma membrane

7. Antigen-
binding site
Antigen-
binding
site
Disulfide
bridge V V V
Light
chain V
Variable regions C C C C
Constant regions
Transmembrane
region
Plasma
membrane
Heavy chains
B cell
Cytoplasm of B cell
A B cell receptor consists of two identical heavy chains and two identical light chains linked by several disulfide bridges.

8. T Cell Receptors for Antigens and the Role of the MHC
Each T cell receptor consists of two different polypeptide chains
Antigen-
binding
site
Variable regions V V
Constant regions C C
Transmembrane
region
Plasma
membrane
 chain
b chain
Disulfide bridge
Cytoplasm of T cell
T cell
A T cell receptor consists of one a chain and one b chain linked by a disulfide bridge.

9. T cells bind to antigen fragments that are bound to cell-surface proteins called MHC molecules
MHC molecules are so named because they are encoded by a family of genes called the major histocompatibility complex
Serve to distinguish one individual from another  recognition of “self” and “non-self”

10. Infected cells produce MHC molecules, which bind to antigen fragments and are transported to the cell surface, a process called antigen presentation
A nearby T cell can then detect the antigen fragment displayed on the cell’s surface
Depending on their source, peptide antigens are handled by different classes of MHC molecules

11. Class I MHC molecules are found on almost all nucleated cells of the body
They display peptide antigens to cytotoxic T cells

12. Class II MHC molecules are located mainly on dendritic cells, macrophages, and B cells
They display antigens to helper T cells

13. Infected cell Microbe Antigen- presenting cell Antigen fragment
LE 43-9
Infected cell
Microbe
Antigen-
presenting
cell
Antigen
fragment
Antigen
fragment
Class I MHC
molecule
Class II MHC
molecule
T cell
receptor
T cell
receptor
Cytotoxic T cell
Helper T cell

14. Clonal Selection of Lymphocytes
In a primary immune response, binding of antigen to a mature lymphocyte induces the lymphocyte’s proliferation and differentiation
This process is called clonal selection
Clonal selection of B cells generates a clone of short-lived activated effector cells and a clone of long-lived memory cells

15. Antigen molecules
B cells that
differ in
antigen
specificity
Antigen
receptor
Antibody
molecules
Clone of memory cells
Clone of plasma cells

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17. Antibody concentration
In the secondary immune response, memory cells facilitate a faster, more efficient response
Second exposure
to antigen X
(secondary response)
104
First exposure
to antigen X
(primary response)
103
Antibodies
to antigen X
Antibody concentration
(arbitrary units)
102
Antibodies
to antigen Z
101
100 7 14 21 28 35 42 49 56
Time (days)

18. Concept 43.3: Humoral and cell-mediated immunity defend against different types of threats
Humoral immune response involves activation and clonal selection of B cells, resulting in production of secreted antibodies
Cell-mediated immune response involves activation and clonal selection of cytotoxic T cells

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

20. Helper T Cells: A Response to Nearly All Antigens
A surface protein called CD4 binds the class II MHC molecule
This binding keeps the helper T cell joined to the antigen-presenting cell while activation occurs
Activated helper T cells secrete cytokines that stimulate other lymphocytes

21. Peptide antigen
Dendritic
cell
Cytotoxic T cell
Class II MHC
molecule
Helper T cell
Cell-mediated
immunity
(attack on
infected cells)
Bacterium
TCR
Humoral
immunity
(secretion of
antibodies by
plasma cells)
CD4
Dendritic
cell
Cytokines
B cell

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23. Cytotoxic T Cells: A Response to Infected Cells and Cancer Cells
Cytotoxic T cells make CD8, a surface protein that greatly enhances interaction between a target cell and a cytotoxic T cell
Binding to a class I MHC complex on an infected cell activates a cytotoxic T cell and makes it an active killer
The activated cytotoxic T cell secretes proteins that destroy the infected target cell

24. Released cytotoxic T cell Cytotoxic T cell Perforin Cancer cell
Granzymes
TCR
CD8
Apoptotic
target cell
Class I MHC
molecule
Pore
Activation of B cells is aided by cytokines and antigen binding to helper T cells
Clonal selection of B cells generates antibody-secreting plasma cells, the effector cells of humoral immunity
Target
cell
Peptide
antigen
Cytotoxic
T cell

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26. LE 43-17 Macrophage Bacterium Peptide antigen B cell Class II MHC
molecule
Secreted antibody
molecules
Clone of plasma cells
TCR
CD4 +
Endoplasmic
reticulum of
plasma cell
Cytokines
Helper T cell
Activated
helper T cell
Clone of memory
B cells