Principles of Adaptive Immunity 3/25/14 12:10 PM
Main difference from innate immunity- cell surface receptor
• One type (B or T Cell receptors)
• Specific for one pathogen.
• Receptor selection is selected and expanded.
Structure of surface of immunoglobulin, AB, and T- cell receptor
• Ig- B cell receptor
o Soluble form= antibody
o Antigen binding site in light chain.
§ highly variable regions
o Ig domain- small modular protein domain used in many
immune system receptors.
• TCR’s: Do not have a soluble form.
o Antigen binding site has highly variable regions
• Specific to their target pathogen.
Gene rearrangement is a mechanism for generating diversity.
• Gene segments- V, J, D
• Rearrangements make functional proteins
o Only happen in B and T cells
Clonal selection- guiding principle of adaptive immunity
• Clonal selection- lymphocyte selection
• Clonal expansion- only a few lymphocytes will bind the pathogen.
Initiating the adaptive immune response
• Dendritic cells links innate and adaptive immune response
o Moves to lymph node to initiate adaptive responseà adaptive
effector cells move to infection site.
• Rare T cells bind antigenà become effector T cellsà activate B cells
• T Cells recognize degraded protein fragments
o Linear peptides from pathogens.
o Peptides are binded to MHC moleculesà TCR’s bind to
peptide-MHC complex on dendritic cell surface.
• Major histocompatibility complex (MHC) molecules
o Loaded with antigen peptides and present them to TCRs o The ligand for TCR
o Stretch peptides out in a linear fashion
o Antigen Presenting cells carry peptide-MHC complex on
o Two classes*********
§ Class 1 present antigens from intracellular pathways
ú Peptides degraded in cytosol
ú Occurs in all cells in the body***
ú CD8 binds MHC
ú Present antigens to cytotoxic t cells that defend
against intracellular pathogens.
• Viral or bacterial intracellular-peptides
§ Class 2 present antigens from extracellular pathogens
ú Peptides degraded in endocytic vesicles
ú Only on APCs (dendritic cells, macrophages, and B
ú CD4 binds MHC
• Present antigens to helper T cells
o Enhance phagocytosis and train B
o Activate B cells to make antibodies.
o Active cytokines and neutrophils.
• Adaptive Immunity cont. 3/25/14 12:10 PM
Antigen presenting Cells
• Activating B Cells to produce Antibodies (Abs).
o Plasma cellsàantibody factories
• Abs are released into circulation or mucosal surfaces.
o Primary function is to mark pathogens for destruction by
phagocyte or other effector cell.
• B cell receptors can bind any macromolecules and native
o T cell receptors can only recognize degraded peptides in MHC
Helper T Cells help B cells to become Abs factories
• Pathogens in Lymph node bind to native B cellsà undergoes
receptor- mediated endocytosis.
• Pathogen degraded peptides are presented via MHC class II.
• Helper T cells that interac