-if innate immunity can’t eliminate infection → adaptive immune system is activated
last line of defense
-2 main weapons
antibodies
produced by B cells
main defense against extracellular bacteria
protect against infection by viruses
T cells
-derived from common lymphoid progenitors in bone marrow
-remain in bone marrow throughout development and maturation process
-B cells begin expressing B cell receptor (BCR/Igα/Igβ co-stimulatory proteins)
responsible for antigen binding
determines antigen specificity of B cell
Igα/Igβ subunit responsible for sending signals to inside of cell once antigen has been bound
-if BCR of immature B cell binds an antigen while still in bone marrow
Igα/Igβ co-stimulatory proteins sends signal to inside of cell causing cell to die
-clonal deletion
process of eliminating self-reactive B cells
analogous to negative selection that thymocytes undergo in thymus
-BCRs don’t need to interact with processed proteins and MHC like TCR of T cells
no similar process of positive selection
-undeleted B cells develop to mature B cells that leave bone marrow
circulates in blood
migrates to spleen and lymph nodes
-BCR of mature, naive B cell is
membrane bound immunoglobulin M (mlgM)
OR
membrane bound immunoglobulin D (mlgD)
-each BCR consists of 4 polypeptide chains
2 identical heavy (H) chains
2 identical light (L) chains
-each immunoglobulin H and L chains has
1 variable (V) region
1 constant (C) region
-VH (variable region of heavy chain) & VL (variable region of light chain) regions combine to form an antigen-binding pocket
2 antigen-binding sites per BCR
-expression of BCR involves rearrangement of immunoglobulin gene segments during development to form complete genes for H & L chain
-single mature B cell has ~100 000 BCRs in membrane
all BCRs on a single B cell
-all BCRs have same antigen specificity
mlgM and mlgD have same VH & VL regions
single cell will bind only one or few similar epitopes (antigenic determinants)
-different B cells have different VH & VL regions in BCRs → different antigen specificity
because different gene segments used when constructing gene for H and L chains
-set of proteins made and secreted by B lymphocytes
-antibodies circulate in blood and other body fluids (mucus)
-immunity due to antibodies = humoral immunity
-antibodies bind to macromolecules that are foreign to individual
-secreted proteins made by B cells
-H and L chain mRNAs are translated into proteins by membrane-bound ribosomes
-H and L chain polypeptides translocated into ER as they are synthesized
-in ER
2 H & 2 L chains joined by disulfide bonds & assembled into complete antibody molecule
-completed antibodies move from ER through Golgi apparatus by vesicular transport
vesicles bud off from Golgi apparatus and move towards plasma membrane
-membrane-bound immunoglobulin (mlg) vs secreted antibodies (slgs)
amino acid sequences at carboxyl-terminus of H chain
mlg: amino acids are hydrophobic
polypeptide is anchored in membrane when vesicle fuses with plasma membrane
slg: amino acids are hydrophilic
antibodies released into extracellular fluids when vesicles with plasma membrane
peptides associated with carboxyl-terminus of H chain
slg doesn’t associate with Igα/Igβ
-secreted antibodies found mainly in blood
also found in bodily secretions
GI secretions, mucus secretions, tears, saliva
-B cells secrete antibodies with same specificity as their BCR
-BCR is surface bound immunoglobulin (mlg) with associated Igα/Igβ component
antibodies are secreted immunoglobulin (slg)
-BCR is composed of 4 polypeptide chains (2H and 2L) that are connected by disulfide bonds
each polypeptide chain has a variable region and constant region
-VH and VL combine to determine antigen-binding specificity of antibody
-each BCR is bivalent
2 antigen-binding sites