Adaptive immunity

Adaptive Immunity
• Adaptive immunity develops as the need arises.
• Two types:
- Humoral immunity
- Mediated by B cells (B lymphocytes)
- Produce antibodies that directly
antigens of invaders
- Cell-mediated immunity
- Involve T cells (T lymphocytes)
- Control antibody production and can
directly kill infected host cells
• The immune system does not recognize the whole
microbe, but innumerable tiny pieces of it.
- Each small segment of an antigen that elicits an
immune response is called an epitope or an antigenic
Figure 24.2
Antibody Structure and Diversity
• An antibody (immunoglobulin) is a Y-shaped structure
made up of four polypeptides:
- Two large heavy chains and two smaller light
chains, connected by disulfide bonds
Figure 24.7
• An antibody has constant and variable regions.
- Variable regions form the antigen-binding site.
• Functional parts of the antibody can be separated
following certain protease treatments.
Figure 24.9
• All antibody
isotypes have the
same basic
- However, each
has a unique
“super” structure.
Figure 24.11
Time Course of an Immune Response
Primary vs. Secondary Antibody Responses
Figure 24.13
Levels of the Immune Response
Levels of the Immune Response
Adaptive Responses Are Specific to
Individual Molecules
 Antigen: the molecule recognized by the
 Each cell can only recognize one antigen
• B cells differentiate by clonal selection.
- When a B cell contacts its cognate antigen, it is stimulated
to proliferate and differentiate into plasma cells (secrete
antibodies) and memory cells.
Figure 24.14
Basic Steps in Antibody Formation
Figure 24.17
The Major Histocompatibility Complex
• Consists of membrane proteins with variable regions
that can bind antigens:
- Class I MHC: Found on all nucleated cells
- Class II MHC: Found only on antigen-presenting cells
Figure 24.20
Figure 24.21
T Cell Receptors
• TCRs associatd with CD3 proteins on cell surface
- Bind antigens only if attached to MHC
- Complex transduces signal into cell, triggering Tcell proliferation.
Figure 24.22
Cell-Mediated Immune Response
• Animation: The Activation of the Humoral and
Cell-Mediated Pathways
Click box to launch animation
Figure 24.24
Figure 24.28
Figure 24.30