Immunology exam 2
Ch. 4 – ANTIBODY STRUCTURE & GENERATION OF B-CELL DIVERSITY
Antibodies (Ab):
- bind to antigens (Ag) (proteins & carbohydrates)
- clear most infections in adaptive response
- in blood, lymph and blood
- secreted by plasma cells (effector B cell)
Only job of B cells: B cell encountering antigen of pathogen stimulates B cell to give rise to
ab-secreting plasma cells, which secrete ab of the same antigen specificity as that of the antigen
receptor of their B-cell precursor.
Ab STRUCTURE:
glycoproteins made of polypeptide chains
2 identical heavy chains (H)
2 identical light chains (L), each linked to
a heavy chain in each arm
AA seq varies at the C terminus (Variable
region/V region). → Identical in both arms
→ Ag binding site
Remaining parts are the constant region.
Disulfide bonds allow them to have a
flexible hinge to allow them to bind in
many different arrangements to Ag.
Antibody (Ab) = secreted/ free/not bound to anything
Immunoglobulin (Ig) = bound to B cell
Antigens (Ag):
- Epitope = part of the Ag that the Ab binds to
1. Linear (epitope is several aa in a straight protein seq.)
2. Discontinuous (epitope is separated parts of the Ag seq. but
conformationally close)
“with viruses you get a mutation on the Ag & it changes the
conformation of the folding so no we don't have Ab binding anymore”
- Binding of Ag to Ab dependent on non-covalent forces
ANTIBODY DIVERSITY
- Ig genes are only expressed in B cells (fragmented in all other cell types)
- These consist of families of gene segments sequentially arranged in chromosome
→ L (leader peptide), V (variable) region, & C (constant) region
- After rearrangements, H & L chains are produced and bound to B cell surface membrane
- V (variable) region: encoded by V light and V heavy gene segments that are selected between many
and rearranged in exons. 2 types of genes segments: V (variable) & J (joining) segments
- Heavy chain genes include also diversity (D) gene segments
, - Somatic recombination: V,D & J segments are randomly cut and re-spliced (L chain single
recombination (V with J) while Heavy chain 2 recominations → D with J, followed by DJ with V)
(see fig. 4.16, slide19)
- done by enzymes: recombination-activating genes (RAG 1 & RAG2)which
bind to recombination signal sequences (RSS) close to each segment
- this way diverse Ag binding sites are made
- Junctional Diversity: RAG adds random nucleotides (not coded in DNA) to coding J (joints) →
palindromic (P-nucleotides) and random (N-nucleotides). This increases diversity.
- For Ab to be secreted they need an SC (secreted seq.) to be secreted in system
- Naive B cells (that haven’t encountered Ag yet) secret IgM & IgD on cell surface (same variable
region but diff. constant region???)
1. Ig chains enter ER and associate to each other attached to ER membrane
2. They associated with Igalpha & Igbeta to be transported to cell surface
3. Travel to surface & help form the Ag receptor
( since their tails stick into cytoplasm Igalpha & Igbeta help with signal transduction for B cell to
differentiate into plasma cell to make + Ab when Ig encounters Ag)
- Somatic Hypermutation: further diversification of Ab’s Variable region (point mutation introduction in
H & L variable regions = higher affinity to pathogen’s aAg)
- dependent on AID (activation-induced cytidine deaminase)
- makes B cells with mutated Ig which might higher affinity to Ag than
original sequence = positive selection
- affinity maturation (as adaptive response precedes, Ab get higher affinity
for pathogen)
- Isotype switching: produces Ig with different constant region but identical Ag specificity (variable
region) by recombination within C genes clusters. Regulated by T cells cytokines during active immune
response
IG CLASSES
5 classes of immunoglobulin: diff. based on carbohydrate distribution on heavy chain
IgG = everywhere your blood is IgA = milk, saliva, sweat & tears, dimeric in gastrointestinal tract
(pentameric) IgM = everywhere your blood is? IgE = skin & mucosal surfaces
IgD = Upper airways, specially tonsils
Ab effector functions to clear pathogens by:
1. NEUTRALIZATION (direct inactivation)
2. OPSONIZATION (coating of pathogen for phagocytosis) → IgA
Ch. 4 – ANTIBODY STRUCTURE & GENERATION OF B-CELL DIVERSITY
Antibodies (Ab):
- bind to antigens (Ag) (proteins & carbohydrates)
- clear most infections in adaptive response
- in blood, lymph and blood
- secreted by plasma cells (effector B cell)
Only job of B cells: B cell encountering antigen of pathogen stimulates B cell to give rise to
ab-secreting plasma cells, which secrete ab of the same antigen specificity as that of the antigen
receptor of their B-cell precursor.
Ab STRUCTURE:
glycoproteins made of polypeptide chains
2 identical heavy chains (H)
2 identical light chains (L), each linked to
a heavy chain in each arm
AA seq varies at the C terminus (Variable
region/V region). → Identical in both arms
→ Ag binding site
Remaining parts are the constant region.
Disulfide bonds allow them to have a
flexible hinge to allow them to bind in
many different arrangements to Ag.
Antibody (Ab) = secreted/ free/not bound to anything
Immunoglobulin (Ig) = bound to B cell
Antigens (Ag):
- Epitope = part of the Ag that the Ab binds to
1. Linear (epitope is several aa in a straight protein seq.)
2. Discontinuous (epitope is separated parts of the Ag seq. but
conformationally close)
“with viruses you get a mutation on the Ag & it changes the
conformation of the folding so no we don't have Ab binding anymore”
- Binding of Ag to Ab dependent on non-covalent forces
ANTIBODY DIVERSITY
- Ig genes are only expressed in B cells (fragmented in all other cell types)
- These consist of families of gene segments sequentially arranged in chromosome
→ L (leader peptide), V (variable) region, & C (constant) region
- After rearrangements, H & L chains are produced and bound to B cell surface membrane
- V (variable) region: encoded by V light and V heavy gene segments that are selected between many
and rearranged in exons. 2 types of genes segments: V (variable) & J (joining) segments
- Heavy chain genes include also diversity (D) gene segments
, - Somatic recombination: V,D & J segments are randomly cut and re-spliced (L chain single
recombination (V with J) while Heavy chain 2 recominations → D with J, followed by DJ with V)
(see fig. 4.16, slide19)
- done by enzymes: recombination-activating genes (RAG 1 & RAG2)which
bind to recombination signal sequences (RSS) close to each segment
- this way diverse Ag binding sites are made
- Junctional Diversity: RAG adds random nucleotides (not coded in DNA) to coding J (joints) →
palindromic (P-nucleotides) and random (N-nucleotides). This increases diversity.
- For Ab to be secreted they need an SC (secreted seq.) to be secreted in system
- Naive B cells (that haven’t encountered Ag yet) secret IgM & IgD on cell surface (same variable
region but diff. constant region???)
1. Ig chains enter ER and associate to each other attached to ER membrane
2. They associated with Igalpha & Igbeta to be transported to cell surface
3. Travel to surface & help form the Ag receptor
( since their tails stick into cytoplasm Igalpha & Igbeta help with signal transduction for B cell to
differentiate into plasma cell to make + Ab when Ig encounters Ag)
- Somatic Hypermutation: further diversification of Ab’s Variable region (point mutation introduction in
H & L variable regions = higher affinity to pathogen’s aAg)
- dependent on AID (activation-induced cytidine deaminase)
- makes B cells with mutated Ig which might higher affinity to Ag than
original sequence = positive selection
- affinity maturation (as adaptive response precedes, Ab get higher affinity
for pathogen)
- Isotype switching: produces Ig with different constant region but identical Ag specificity (variable
region) by recombination within C genes clusters. Regulated by T cells cytokines during active immune
response
IG CLASSES
5 classes of immunoglobulin: diff. based on carbohydrate distribution on heavy chain
IgG = everywhere your blood is IgA = milk, saliva, sweat & tears, dimeric in gastrointestinal tract
(pentameric) IgM = everywhere your blood is? IgE = skin & mucosal surfaces
IgD = Upper airways, specially tonsils
Ab effector functions to clear pathogens by:
1. NEUTRALIZATION (direct inactivation)
2. OPSONIZATION (coating of pathogen for phagocytosis) → IgA
