Transcript Adaptive Immunity

Adaptive/Acquired Immunity
DENT 516
Adaptive immunity
Refers to antigen-specific defense
mechanisms that take several days to
become protective and are designed to
remove a specific antigen. This is the
immunity one develops throughout life.
There are two major branches of the
adaptive immune responses: humoral
immunity and cell-mediated immunity.
Humoral Immunity
• humoral immunity involves the production of
antibody molecules in response to an antigen and
is mediated by B-lymphocytes.
Cell Mediated Immunity
 CMI involves production of cytotoxic T-lymphocytes,
activated macrophages, activated NK cells and
cytokines in response to an antigen and is mediated
by T-lymphocytes.
Antigens
• Ags are molecules that elicit an immune
response in the body
• Ags can be:
– Proteins
– Polysaccharides
– Conjugates of lipids with
• Proteins (lipoproteins)
• Polysaccharides (glycolipids)
Antigens II
• Ags that enter body from environment include:
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Inhaled macromolecules; e.g., cat hair proteins
asthma
Ingested macromolecules; e.g. shellfish proteins
allergy
Molecules introduced beneath the skin; e.g. splinter, vaccine
Exogenous antigens
 Ags generated within the cells of the host:
 Proteins encoded by viral genes that have infected a cell
 Aberrant proteins that are encoded by mutant genes; e.g.
proteins from mutated genes in cancer cells
 Endogenous antigens
Antigen Presentation
Initial immune response to any
Ag requires the Ag be recognized
by a T cell.
This is best exemplified by AIDS
w/loss of CD4+ T cells.
Dendritic cell
B cell
Two categories of Ags are
processed and presented to T cells
by different mechanisms:
Exogenous Ags
Endogenous Ags
T cell activated; cytokines released;
Th1, activate M; Th2, activate B cells
Exogenous antigens
• E. Ag’s (inhaled, ingested, injected), taken up
by APCs:
– Phagocytic cells; dendritic cells, macrophages
– B lymphocytes (produce antibodies)
• APCs
 engulf Ags by endocytosis (endosome-lysosome)
 Ag degraded into short peptides
 peptides displayed at cell surface nestled w/i
a class II histocompatibility molecule
 recognized by CD4+ T cells
Endogenous Antigens
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Ags that are generated w/i a cell; e.g.,
Mt bug, viral proteins in infected cells
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Peptides displayed at cell surface
nestled w/I a class I histocompatibility
molecule
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Recognized by CD8+ T cells
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CD8+ T cells are cytotoxic
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Have machinery to destroy infected cell
T cell activated; kills infected cell
B Lymphocytes: A Special Case
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Process Ag by MHC II pathway but:
B cells engulf Ag by receptor mediated
endocytosis
BCRs are surface antibodies anchored in
plasma membrane
Affinity of BCR for an Ag epitope is so
high that the B cell can internalize the Ag
at concentrations thousands of times
smaller than needed for a macrophage
CD4+ T cell recognizes displayed Ag and
is stimulated to release cytokines
These stimulate B cells to grow into a
clone of cells; plasma cells
These plasma cells synthesize BCRs with
identical binding site for the Ag epitope but
w/o the transmembrane tail
The Abs are secreted
release Th2 cytokines
Clonal Expansion
Types of T cells