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Adaptive/Acquired Immunity
DENT 516
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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.
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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.
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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)
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Antigens II
• Ags that enter body from environment include:– 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
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Antigen PresentationInitial 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.
Two categories of Ags are processed and presented to T cells by different mechanisms:
Exogenous Ags
Endogenous Ags
Dendritic cell
B cell
T cell activated; cytokines released;
Th1, activate M; Th2, activate B cells
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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
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Endogenous Antigens• Ags that are generated w/i a cell; e.g.,
Mt bug, viral proteins in infected cells
• Peptides displayed at cell surface nestled w/I a class I histocompatibility molecule
• Recognized by CD8+ T cells
• CD8+ T cells are cytotoxic
• Have machinery to destroy infected cell
T cell activated; kills infected cell
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B Lymphocytes: A Special Case• 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
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Clonal Expansion
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Types of T cells
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