Humoral Immunity & Immunoglobulin Structure and Function Dr. Adel Almogren.
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Transcript of Humoral Immunity & Immunoglobulin Structure and Function Dr. Adel Almogren.
Humoral Immunity &Immunoglobulin Structure and
Function
Dr. Adel Almogren
Humoral (Humoral (Antibody-Mediated) Immunity) Immunity + +Involves production of antibodies against foreign Involves production of antibodies against foreign
antigensantigens..
+ +Antibodies are produced by a subset of lymphocytes Antibodies are produced by a subset of lymphocytes called called B cellsB cells..
+ +B cells that are stimulated will actively secrete B cells that are stimulated will actively secrete antibodies and are called antibodies and are called plasma cellsplasma cells..
+ +Antibodies are found in Antibodies are found in extracellular fluidsextracellular fluids (blood (blood plasma, lymph, mucus, etc.) and the surface of B cellsplasma, lymph, mucus, etc.) and the surface of B cells..
+ +Defense against bacteria, bacterial toxins, and Defense against bacteria, bacterial toxins, and viruses that circulate freely in body fluids, viruses that circulate freely in body fluids, beforebefore they they enter cellsenter cells..
+ +Also cause certain reactions against transplanted Also cause certain reactions against transplanted tissuetissue..
How Do B Cells Produce Antibodies?How Do B Cells Produce Antibodies? B cells develop from B cells develop from stem cellsstem cells in the bone marrow in the bone marrow
of adults (liver of fetuses).of adults (liver of fetuses).
After maturation B cells migrate to lymphoid organs After maturation B cells migrate to lymphoid organs (lymph node or spleen). (lymph node or spleen).
Clonal SelectionClonal Selection: When a B cell encounters an : When a B cell encounters an
antigen it recognizes, it is stimulated and divides into antigen it recognizes, it is stimulated and divides into
many clones called many clones called plasma cellsplasma cells, which actively , which actively
secrete antibodies, and secrete antibodies, and memory B cellsmemory B cells
Each B cell produces antibodies that will recognize Each B cell produces antibodies that will recognize only one antigenic determinant.only one antigenic determinant.
Clonal Selection of B Cells is Caused by Antigenic Stimulation
Humoral Immunity (Continued)Humoral Immunity (Continued)
Clonal SelectionClonal Selection Clonal SelectionClonal Selection: B cells that encounter : B cells that encounter
stimulating antigen will proliferate into a large stimulating antigen will proliferate into a large group of cells (also apply to T cells).group of cells (also apply to T cells).
Why donWhy don’’t we produce antibodies against our t we produce antibodies against our own antigens? We have developed own antigens? We have developed tolerancetolerance to to them.them.
Clonal DeletionClonal Deletion: B (and T) cells that react against : B (and T) cells that react against selfself antigens appear to be destroyed during fetal antigens appear to be destroyed during fetal development. Process is poorly understood.development. Process is poorly understood.
Antibody ProductionAntibody ProductionTT--Dependent Antigens:Dependent Antigens:
Antibody production Antibody production requiresrequires assistance from T helper cells.assistance from T helper cells. A macrophage cells ingest antigen and presents it to TA macrophage cells ingest antigen and presents it to THH cell.cell. TTHH cell stimulates B cells specific for antigen to become cell stimulates B cells specific for antigen to become
plasma cells.plasma cells. Antigens are mainly proteins on viruses, bacteria, foreign red Antigens are mainly proteins on viruses, bacteria, foreign red
blood cells, and haptenblood cells, and hapten--carrier molecules.carrier molecules.TT--Independent Antigens:Independent Antigens:
Antibody production does not require assistance from T cells.Antibody production does not require assistance from T cells. Antigens are mainly polysaccharides or lipopolysaccharides Antigens are mainly polysaccharides or lipopolysaccharides
with repeating subunits (bacterial capsules).with repeating subunits (bacterial capsules). Weaker immune response than for TWeaker immune response than for T--dependent antigens.dependent antigens.
B cell activation
T-independent antibody response generally have1. no memory2. no isotype switching3. no somatic mutations
Thymus-dependent because T cells are required
Thymus-independent because T cells are not needed
Some responses require T help whereas other do not
CL VL
S
S
S
S
SS
SS
CH3
CH2 CH1
VH
Fc Fab
F(ab)2
Domains are folded, compact, protease resistant structures
Domain Structure of Immunoglobulins
Pepsin cleavage sites - 1 x (Fab)2 & 1 x FcPapain cleavage sites - 2 x Fab 1 x Fc
Light chain Cdomains or
Heavy chain Cdomains
or
CH3
CH3
CH2
CH3
CH2
CH1
CH3
CH2
CH1VH1
CH3
CH2
CH1VH1
CL
CH3
CH2
CH1VH1
CL
VL
CH3
CH2
CH1VH1
CLVL
Hinge
CH3
CH2
CH1VH
CL VL
Elbow
FbFv
CH3
CH2
Fb
Fv
Fv
FvHinge
Elbow
CH3
CH2
Fb
Fv
Flexibility andmotion of immunoglobulins
Hypervariable regions
FR1 FR2 FR3 FR4CDR2 CDR3CDR1
Amino acid No.
Variability80
100
60
40
20
20 40 60 80 100 120
•Most hypervariable regions coincided with antigen contact points -
the COMPLEMENTARITY DETERMINING REGIONS (CDRs)
Hypervariable CDRs are locatedon loops at the end of the Fv regions
•The sequences of the hypervariable loops are highly variable
amongst antibodies of different specificities
•Variable amino acid sequence in the
hypervariable loops accounts for the diversity of
antigens that can be recognised by a repertoire of
antibodies
Hypervariable loops and framework: Summary
Antibody + complement- mediated damage to E. coli
Healthy E. coli
Electron micrographs of the effect of antibodies and complement upon bacteria
Structure and function of the Fc region
CH3
CH2
IgA IgD IgG
CH4
CH3
CH2
IgE IgMThe hinge region is replaced by an additional Ig domain
Fc structure is common to all specificities of antibody within an ISOTYPE(although there are allotypes)
The structure acts as a receptor for complement proteins and a ligand for cellular binding sites
Monomeric IgM
IgM only exists as a monomer on the surface of B cells
Monomeric IgM has a very low affinity for antigen
C4
C3C2 C1
N.B. Only constant heavy chain
domains are shown
IgM forms pentamers and hexamers
CC
C
C
C C
Multimerisation of IgM
C4 C
3
C2
C
C
C4
C3
C2
C CC4
C3
C2
C
C
C4
C3C2
C
C
C4
C3
C2
C
C
s s
ss
ss
C
Css
IgM facts and figures
Heavy chain: - Mu
Half-life: 5 to 10 days
% of Ig in serum: 10
Serum level (mgml-1): 0.25 - 3.1
Complement activation: ++++ by classical pathway
Interactions with cells: Phagocytes via C3b receptors
Epithelial cells via polymeric Ig receptor
Transplacental transfer: No
Affinity for antigen: Monomeric IgM - low affinity - valency of 2
Pentameric IgM - high avidity - valency of 10
IgD facts and figures
??IgD & IgM ??
Heavy chain: - Delta
Half-life: 2 to 8 days
% of Ig in serum: 0.2
Serum level (mgml-1): 0.03 - 0.4
Complement activation: No
Interactions with cells: T cells via lectin like IgD receptor
Transplacental transfer: No
IgA dimerisation and secretion
IgA is the major isotype of antibody secreted at mucosal surfaces
Exists in serum as a monomer, but more usually as a J chain-
linked dimer, that is formed in a similar manner to IgM pentamers.
JC C
SS
SS
C
C
SS
SS
C
C
s s
IgA exists in two subclasses
IgA1 is mostly found in serum and made by bone marrow B cells
IgA2 is found in higher concentration in mucosal secretions, colostrum
Epithelialcell
JC C
SS
SS
C
C
SS
SS
CC
ss
Secretory IgA and transcytosis
B
JC C
SS
SS
CC
SS
SS
CCss
JC C
SS
SS
C
C
SS
SS
CC
ss
JC C
SS
SS
C
C
SS
SS
CC
ss
pIgR & IgA areinternalised
‘Stalk’ of the pIgR is degraded to release IgA containing part of the pIgR - the secretory component
JC C
SS
SS
C
C
SS
SS
CC
ss
IgA and pIgR are transported to the apical surface in vesicles
B cells located in the submucosaproduce dimeric IgA
Polymeric Ig receptors are expressed on the basolateral surface of epithelial cells to capture IgA produced in the mucosa
IgA facts and figures
Heavy chains: 1or2 - Alpha 1 or 2
Half-life: IgA1 5 - 7 daysIgA2 4 - 6 days
Serum levels (mgml-1): IgA1 1.4 - 4.2IgA2 0.2 - 0.5
% of Ig in serum: IgA1 11 - 14
IgA2 1 - 4
Complement activation: IgA1 - by alternative and lectin pathwayIgA2 - No
Interactions with cells: Epithelial cells by pIgRPhagocytes by IgA receptor
Transplacental transfer: No
IgE facts and figures
its role in protecting against parasitic infectionsIgE is also closely linked with allergic diseases
Heavy chain: - Epsilon
Half-life: 1 - 5 days
Serum level (mgml-1): 0.0001 - 0.0002
% of Ig in serum: 0.004
Complement activation: No
Interactions with cells: Via high affinity IgE receptors expressed by mast cells, eosinophils, basophils and Langerhans cellsVia low affinity IgE receptor on B cells and monocytes
Transplacental transfer: No
IgG facts and figures
Heavy chains: 123 4 - Gamma 1 - 4
Half-life: IgG1 21 - 24 days IgG2 21 - 24 days
IgG3 7 - 8 days IgG4 21 - 24 days
Serum level (mgml-1): IgG1 5 - 12 IgG2 2 - 6IgG3 0.5 - 1 IgG4 0.2 - 1
% of Ig in serum: IgG1 45 - 53 IgG2 11 - 15IgG3 3 - 6 IgG4 1 - 4
Complement activation: IgG1 +++ IgG2 + IgG3 ++++ IgG4 No
Interactions with cells: All subclasses via IgG receptors on macrophages and phagocytes
Transplacental transfer: IgG1 ++ IgG2 +IgG3 ++ IgG4 ++
The neonatal Fc receptor may be responsible!
Carbohydrate is essential for complement activation
Subltly different hinge regions between subclasses accounts for differing abilities to activate complement
C1q binding motif is located on the C2 domain
Fc receptors
Receptor Cell type Effect of ligation
FcRI Macrophages Neutrophils,
Eosinophils, Dendritic cells Uptake, Respiratory burst
FcRIIA Macrophages Neutrophils,
Eosinophils, Platelets
Langerhans cells Uptake, Granule release
FcRIIB1 B cells, Mast Cells No Uptake, Inhibition of stimulation
FcRIIB2 Macrophages Neutrophils,
Eosinophils Uptake, Inhibition of stimulation
FcRIII NK cells, Eosinophils,
Macrophages, Neutrophils
Mast cells Induction of killing (NK cells)
Hinge
Fv
Fb
Fab
CH3
CH2
CH1VH1
CL VL
Fc
Elbow
Carbohydrate
Antibody Dependent Cell Mediated Cytotoxicity (ADCC)
Target cell is covered with antibodies, Target cell is covered with antibodies, leaving Fc portion sticking outwardsleaving Fc portion sticking outwards..
Natural killer and other nonspecific cells Natural killer and other nonspecific cells that have receptors for Fc region are that have receptors for Fc region are stimulated to kill targeted cellsstimulated to kill targeted cells..
Target organism is lysed by substances Target organism is lysed by substances secreted by attacking cellssecreted by attacking cells..
Used to destroy large organisms that Used to destroy large organisms that cannot be phagocytosedcannot be phagocytosed..
Destruction of Large Parasites by ADCC