chapter 5 immunoglobulin
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Chapter 5Chapter 5
ImmunoglobulinImmunoglobulin
ContentsIntroductionSectionⅠ Molecular Structure of IgSectionⅡ Characteristics and Functions of the 5 Classes of IgSection Ⅲ Fc Receptors for Ab MoleculesSection Ⅳ Biological Activity of AbSectionⅤ Immunogenicity of IgSection Ⅵ Artificial Ab
Concepts Antibody (Ab): Glycoprotein molecule
s that are produced by plasma cells and can combine with the corresponding Ag specifically are called Ab.
Ab is produced by B cells in the response to a stimulation of Ag.
Ab possesses a high degree of specificity and affinity
• Immunoglobulin (Ig): It refers to all globulins that possess the activ
ity of Ab or show a similar structure to Ab
• Therefore, All Abs are Igs, but not all Igs possess the functions of Abs
Other Concepts
γ- Globulin Antiserum Humoral Immunity sIg and mIg(BCR)
SectionⅠ Molecular Structure of Ig
Ⅰ. Basic structure Ig is composed of four polypeptide chains j
oined by S-S bonds. inter-chain disulfide bonds (S-S) intra-chain disulfide bonds (S-S)
It shows “T” or “Y” shape.
1. H and L chain:
. Heavy chains (H): 450 ~ 550 aa, 50 ~ 75 KD
. Light chains (L): 214 aa, 25 KD
Two terminal ends for each peptide chain
“N” terminal end “C” terminal end
L chains attach to H chains from “N” end
“N”
“C”
2. classes and types of Ig (1) According to the differences of H chains (amino acid composition, sequence) Igs can be divided into 5 classes • Five classes of H Chain: • Five classes of Igs: IgG IgA IgM IgD IgE
subclasses
IgG1~ IgG4 IgA1, IgA2
(2) According to the differences of L chains:
Two types of L chain: ,
: 20:1 (in mice); 2:1 (in
human)
1~ 4
3. Two regions of each peptide chain
(1) Constant region (C)
(3)(3) Hinge regionHinge region
(2) Variable(2) Variable regionregion (V) (V)
(1) Constant region ( C )• CH: 3/4 or 4/5 (,) of H chain from the c
end
• CL: 1/2 of L chain from the c end
(2) Variable region ( V )• CH: 1/4 or 1/5 (,) of H chain from the N
end
• CL: 1/2 of L chain from the N end
3. Two regions of each peptide chain chain
(2) Variable region ( V ):
Hypervariable region(HVR) There are three highly diversity
stretches within the V egion, they are called HVR.
Framework region(FR): FR1-FR4
Ag-binding sites
Complementarity determining regions(CDR)
(2) Variable region (V)
Complementarity determining regions(CDR) L: CDR1, CDR2, CDR3
H: CDR1, CDR2, CDR3
Idiotype of Ig Igs produced by different B cells possess unique s
tructure respectively in hypervariable region (HVR), the unique structure of Ig is called idiotype or idiotypic determinant
In fact: HVR CDR Idiotype are in the same sites of Ig
(3) Hinge region:• Flexible and suitable for CDR of Ig bi
nding to antigenic determinants.• Sensitive to proteolytic enzyme• IgM, IgE
Other structures of Ig • Joining chain(J) Secretory piece(SP)
Joining chain(J ) :
Produced by plasma cells Functions:linker, to compose dimer 、
pentamer or polymer(IgA, IgM)
Secretory piece( SP): . Produced by mucosa epithelial cells . Secretory IgA (sIgA) . Functions: protect sIgA, resist proteolys
is in extra secretory liquid.
IgA
Ⅱ. Domains of Ig
1. Domain :
Polypeptide chains of Ig are folded into a globul
ar structure by intra chain s-s bond within each 110aa region which is called a domain
2. Domains of Ig
• L chain(2) : VL, CL• H chain(4~5): VH, CH1, CH2, CH3 CH4(in IgM,IgE) hinge region
3. Function of each domain
• VH, VL: antigen-binding site• CH1, CL: allogeneic marker• CH2/CH3: complement-fixing site, permeate placenta(IgG) • CH3/CH4: cell-binding site Hinge region :flexible and suitable for CDR of I
g binding to antigenic determinants
Ⅲ. Hydrolytic fragments of Ig
Ig can be digested by papain and pepsin
• Position • Fragments • Function
1. Digested by papain• Position: near the S-S bonds of H inter-chains fromthe N end
Fragments: 2Fab:fragment antigen-binding Fc:fragment crystallizable
Function: Fab: recognize and bind Ag
Fc: (1) fix complement (2) crossing the placenta (3) bind to FcR in different cells
•
2. Digested by pepsin• Position: near the S-S bond of H inter-chains from the C end
• Fragments and function : F(ab′)2: bind antigen(2 valence) pFc′: no function
Significance
Elucidating the relationships between the structure and function of Igs
Decrease the immunogenicity of Ig for clinical treatment
SectionⅡ Characteristics and Functions of the 5 Classes of Igs
Ⅰ. IgG 1. Highest concentration in serum (75% of t
otal Ig)
2. Four subclasses: IgG1, IgG2, IgG3, IgG4
3. Unique Ig that can pass through placenta
4. Half-life is longer( 16-24 days )
5. Starts to be produced at 2-3 month after birth and reach the level of adult at 5 years old
6. Functions of IgG:• Against bacteria and virus,neutralize toxin• Combine with the Fc receptor(FcγR)• Activate complement• Combine with SPA• Some belong to the auto-antibodies• Take part in type Ⅱ and Ⅲ hypersensitivi
ty
Ⅱ. IgM 1. Highest MW : pentamer ( 90 KD ) ,10 valence
s
2. Half-life is shorter(4~5 days)
3. The first Ig to be synthesized• Appear in the early stage after infection • Be produced during fetus• The first mIg of the B cells, act as the antigen recept
ors(BCR)
4. Functions:• IgM is more effective in binding Ag and a
ctivating C, and play an important role in anti-infection
• Natural Ab for blood-type antigen • Auto-antibody: rheumatoid factor(RF)• Take part in type Ⅱ and Ⅲ hypersensiti
vity
ⅢⅢ. . IgAIgA
1. Two types Serum type : monomer Secretary type ( sIgA ) : dimer , trimer or polymer
2. Two subclasses : IgA1 , IgA2
3. To be produced at 4 months after birth
4. Exist in almost all body fluid
6. Local mucosal immunity
• Immune barrier • Neutralize virus/toxin• Rich in colostrum • Activate C by alternative pathway• Take part in type Ⅲ hypersensitivity
Ⅳ. IgD1. The concentration in serum is low and
sensitive to proteinase
2. Act as the antigen receptor on B cells (mIgD): Regulate the differentiation of B cells
Ⅴ. IgE1.Concerntration of IgE in serum is the lo
west in normal individual, but is very high in some patients.
2.Related to typeⅠpersensitivity FcεRⅠ: mast cell, basophil
Section Ⅲ Fc Receptors for Ab Molecules
IgG---FcR: FcRⅠ(CD64)---phagocyte FcRⅡ(CD32)---immune complex FcRⅢ(CD16)---NK, MΦ,T cell
IgE---FcR: FcRⅠ--- mast cell, basophil FcRⅡ--- macrophage, B cell
IgA---FcαR(CD89)---phagocyte, neutrophil
SectionⅣ Biological Activity of Ab
1. Recognize and bind to antigen specifically 2. Fix complement3. Bind to Fc receptor on some cells 4. Transfer selectively : .Planceta transfer (IgG) .Mucosa transfer (sIgA)
Affinity and Avidity
Neutralization
IgM,IgG1~3: classical pathway
IgA,IgG4,IgE:
alternative pathway
MAC
(1) Opsonization(IgG, IgM): Enhance the phagocytosis of MΦ
(2) ADCC( antibody dependent cell mediated cytotoxicity)
(3) Hypersensitivity typeⅠ - mast cell, basophil(FcRⅠ)
FcRI
degranulation
IgE
allergen
inflammationinflammation
SectionⅤ Immunogenicity of Ig
Isotype: CH, CL
Allotype: CH, CL
Idiotype: VH, VL
Anti-idiotype antibody
SectionⅥ Artificial Ab
Polyclonal Ab Monoclonal Ab Gene engineering Ab
1. Polyclonal Ab A mixture Ab with different specifici
ties and affinities Generate in a natural response or ar
tificial immunization Cross reaction
Cross-reactivity:if two antigens share an epitopean antibody recognizes an unrelated,but chemically similar, epitope
2. Monoclonal Ab (mAb) Ab produced by single clone (or one hy
bridomas clone ) and having a single specificity
mAb / McAb Prepared by hybridomas technique:
Immunized spleen cells(B) hybride with myeloma cells----hybridomas
Artificial antibodies
Derived from different B Lymphocytes cell lines
POLYCLONAL. MONOCLONAL.
Derived from a single B cell clone
Batch to Batch variation affecting Ab reactivity &
titre
mAb offer Reproducible, Predictable & Potentially
inexhaustible supply of Ab with exquisite specificity
Enable the development of secure immunoassay systems.
NOT Powerful tools for clinical diagnostic tests
3.Gene engineering Ab• Abs prepared by the method of gene r
ecombination
• Chimeric Ab:human Fc bind with mice Fab• Recombinant single chain Ab:VH-linker-VL
Human-mouse chimeric Ab
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