DEFINITION
Immunoglobulin (Ig)
Immunoglobulins are glycoprotein molecules that function as antibodies and are produced by plasma cells in response to an immunogen. The immunoglobulins derive their name from the finding that they migrate in the region of globulins when antibody-containing serum is placed in an electrical field.
Figure- 1-showing the Electrophoretic separation of plasma proteins
BASIC STRUCTURE OF IMMUNOGLOBULINS
Although different immunoglobulins differ structurally but they all are built from the same basic units.(Figure-2)
A. Heavy and Light Chains
All immunoglobulins have a four chain structure as their basic unit. They are composed of two identical light chains (23kD) and two identical heavy chains (50-70kD)
B. Disulfide bonds
1. Inter-chain disulfide bonds - The heavy and light chains and the two heavy chains are held together by inter-chain disulfide bonds and by non-covalent interactions .The number of inter-chain disulfide bonds varies among different immunoglobulin molecules.
2. Intra-chain disulfide binds - Within each of the polypeptide chains there are also intra-chain disulfide bonds.
C. Variable (V) and Constant (C) Regions
Both the heavy and light chain can be divided into two regions based on variability in the amino acid sequences. These are the
1. Light Chain – VL (110 amino acids) and CL (110 amino acids)
2. Heavy Chain – VH (110 amino acids) and CH (330-440 amino acids)
D. Hinge Region
This is the region at which the arms of the antibody molecule form a Y. It is called the hinge region because there is some flexibility in the molecule at this point.
E. Domains
Three dimensional images of the immunoglobulin molecule show that it is not a straight molecule rather, it is folded into globular regions each of which contains an intra-chain disulfide bond (figure-2). These regions are called domains.
1. Light Chain Domains – VL and CL
2. Heavy Chain Domains – VH, CH1,CH2CH3 (or CH4)
F. Oligosaccharides
Carbohydrates are attached to the CH2 domain in most immunoglobulins. However, in some cases carbohydrates may also be attached at other locations.
Figure-2- showing the general structure of Immunoglobulin
GENERAL FUNCTIONS OF IMMUNOGLOBULINS
A. Antigen binding
Immunoglobulins bind specifically to one or a few closely related antigens. Each immunoglobulin actually binds to a specific antigenic determinant. Antigen binding by antibodies is the primary function of antibodies and can result in protection of the host. The valency of antibody refers to the number of antigenic determinants that an individual antibody molecule can bind. The valency of all antibodies is at least two and in some instances more.
Figure-3- Showing the general structure of Immunoglobulin and structure of antigen binding cleft. Antigenic determinants are present on the surface of antigen while the antigen binding sites are present in the antigen binding cleft made by both light chain and heavy chain
B. Effector Functions
Frequently the binding of an antibody to an antigen has no direct biological effect. Rather, the significant biological effects are a consequence of secondary “effector functions” of antibodies. The immunoglobulins mediate a variety of these effector functions. Not every immunoglobulin will mediate all effector functions. Such effector functions include:
1. Fixation of complement - This results in lysis of cells and release of biologically active molecules
2. Binding to various cell types – Phagocytic cells, lymphocytes, platelets, mast cells, and basophils have receptors that bind immunoglobulins. This binding can activate the cells to perform some function.
Figure-4 showing the receptors present on the surface of macrophages to bind the antigen antibody complex
Some immunoglobulins also bind to receptors on placental trophoblasts, which results in transfer of the immunoglobulin across the placenta. As a result, the transferred maternal antibodies provide immunity to the fetus and newborn.
Figure- 5 showing the functions of different regions of the immunoglobulins
IMMUNOGLOBULIN FRAGMENTS: STRUCTURE/FUNCTION RELATIONSHIPS
Immunoglobulin fragments produced by proteolytic digestion -
A. Fab
Digestion with papain breaks the immunoglobulin molecule in the hinge region before the H-H inter-chain disulfide bond Figure 6. This results in the formation of two identical fragments that contain the light chain and the VH and CH1 domains of the heavy chain.
Digestion with papain breaks the immunoglobulin molecule in the hinge region before the H-H inter-chain disulfide bond Figure 6. This results in the formation of two identical fragments that contain the light chain and the VH and CH1 domains of the heavy chain.
Figure-6 showing the site of action of Papain (a proteolytic enzyme). Fab and Fc are the two fragments obtained by proteolytic cleavage.
Antigen binding - These fragments are called the Fab fragments because they contained the antigen binding sites of the antibody. Each Fab fragment is monovalent whereas the original molecule was divalent. The combining site of the antibody is created by both VH and VL.
B. Fc
Digestion with papain also produces a fragment that contains the remainder of the two heavy chains each containing a CH2 and CH3 domain. This fragment was called Fc because it was easily crystallized.
Digestion with papain also produces a fragment that contains the remainder of the two heavy chains each containing a CH2 and CH3 domain. This fragment was called Fc because it was easily crystallized.
Effector functions - The effector functions of immunoglobulins are mediated by this part of the molecule. Different functions are mediated by the different domains in this fragment (figure 5).
Figure-7- showing the site of action of Pepsin on Immunoglobulin.
C. F(ab’)2
Treatment of immunoglobulins with pepsin results in cleavage of the heavy chain after the H-H inter-chain disulfide bonds resulting in a fragment that contains both antigen binding sites (figure 7). This fragment is called F(ab’)2because it is divalent. The Fc region of the molecule is digested into small peptides by pepsin. The F(ab’)2binds antigen but it does not mediate the effector functions of antibodies.
Treatment of immunoglobulins with pepsin results in cleavage of the heavy chain after the H-H inter-chain disulfide bonds resulting in a fragment that contains both antigen binding sites (figure 7). This fragment is called F(ab’)2because it is divalent. The Fc region of the molecule is digested into small peptides by pepsin. The F(ab’)2binds antigen but it does not mediate the effector functions of antibodies.
HUMAN IMMUNOGLOBULIN CLASSES, SUBCLASSES, TYPES AND SUBTYPES
A. Immunoglobulin classes
The immunoglobulins can be divided into five different classes, based on differences in the amino acid sequences in the constant region of the heavy chains. All immunoglobulins within a given class will have very similar heavy chain constant regions.
1. IgG – Gamma heavy chains
2. IgM – Mu heavy chains
3. IgA – Alpha heavy chains
4. IgD – Delta heavy chains
5. IgE – Epsilon heavy chains
Immunoglobulin Subclasses
The classes of immunoglobulins can de divided into subclasses based on small differences in the amino acid sequences in the constant region of the heavy chains. All immunoglobulins within a subclass will have very similar heavy chain constant region amino acid sequences.
1. IgG Subclasses
a) IgG1 – Gamma 1 heavy chains
b) IgG2 – Gamma 2 heavy chains
c) IgG3 – Gamma 3 heavy chains
d) IgG4 – Gamma 4 heavy chains
2. IgA Subclasses
a) IgA1 – Alpha 1 heavy chains
b) IgA2 – Alpha 2 heavy chains
Immunoglobulin Types
Immunoglobulins can also be classified by the type of light chain that they have. Light chain types are based on differences in the amino acid sequence in the constant region of the light chain.
Immunoglobulins can also be classified by the type of light chain that they have. Light chain types are based on differences in the amino acid sequence in the constant region of the light chain.
1. Kappa light chains
2. Lambda light chains
STRUCTURE AND SOME PROPERTIES OF IG CLASSES AND SUBCLASSES
A. IgG
1. Structure
All IgG’s are monomers (7S immunoglobulin). The subclasses differ in the number of disulfide bonds and length of the hinge region.
2. Properties
IgG is the most versatile immunoglobulin because it is capable of carrying out all of the functions of immunoglobulin molecules.
a) IgG is the major Ig in serum – 75% of serum Ig is IgG
b) IgG is the major Ig in extra vascular spaces
c) Placental transfer – IgG is the only class of Ig that crosses the placenta. Transfer is mediated by a receptor on placental cells for the Fc region of IgG. Not all subclasses cross equally well; IgG2 does not cross well.
d) Fixes complement – Not all subclasses fix equally well; IgG4 does not fix complement
e) Binding to cells – Macrophages, monocytes and neutrophils and some lymphocytes have Fc receptors for the Fc region of IgG. A consequence of binding to the Fc receptors on such cells is that the cells can now internalize the antigen better. The antibody prepares the antigen for killing by the phagocytic cells. The term opsonin is used to describe substances that enhance phagocytosis. (Coating of the surface of pathogen by antibody is called opsonization).IgG is a good opsonin. Binding of IgG to Fc receptors on other types of cells results in the activation of other functions.
Figure-8- showing the structure of Ig G
IgM
1. Structure
IgM normally exists as a pentamer (19S immunoglobulin) but it can also exist as a monomer. In the pentameric form all heavy chains are identical and all light chains are identical. Thus, the valence is theoretically 10. IgM has an extra domain on the mu chain (CH4) and it has another protein covalently bound via a S-S bond called the J chain. This chain functions in polymerization of the molecule into a pentamer.
IgM normally exists as a pentamer (19S immunoglobulin) but it can also exist as a monomer. In the pentameric form all heavy chains are identical and all light chains are identical. Thus, the valence is theoretically 10. IgM has an extra domain on the mu chain (CH4) and it has another protein covalently bound via a S-S bond called the J chain. This chain functions in polymerization of the molecule into a pentamer.
2. Properties
a) IgM is the third most common serum Ig.
b) IgM is the first Ig to be made by the fetus and the first Ig to be made by a virgin B cells when it is stimulated by antigen.
c) As a consequence of its pentameric structure, IgM is a good complement fixing Ig. Thus, IgM antibodies are very efficient in leading to the lysis of microorganisms.
d) As a consequence of its structure, IgM is also a good agglutinating Ig . Thus, IgM antibodies are very good in clumping microorganisms for eventual elimination from the body.
e) IgM binds to some cells via Fc receptors.
f) B cell surface Ig
Surface IgM exists as a monomer and lacks J chain but it has an extra 20 amino acids at the C-terminus to anchor it into the membrane . Cell surface IgM functions as a receptor for antigen on B cells.
Figure-9- showing the structure of Ig M
IgA
1. Structure
Serum IgA is a monomer but IgA found in secretions is a dimer as presented in Figure 10. When IgA exits as a dimer, a J chain is associated with it.
When IgA is found in secretions is also has another protein associated with it called the secretory piece or T piece; sIgA is sometimes referred to as 11S immunoglobulin. Unlike the remainder of the IgA which is made in the plasma cell, the secretory piece is made in epithelial cells and is added to the IgA as it passes into the secretions . The secretory piece helps IgA to be transported across mucosa and also protects it from degradation in the secretions.
Figure-10- showing the structure of Ig A
2. Properties
a) IgA is the 2nd most common serum Ig.
b) IgA is the major class of Ig in secretions – tears, saliva, colostrum, mucus. Since it is found in secretions secretory IgA is important in local (mucosal) immunity.
c) Normally IgA does not fix complement, unless aggregated.
d) IgA can binding to some cells – PMN’s and some lymphocytes.
IgD
1. Structure
IgD exists only as a monomer.
2. Properties
a) IgD is found in low levels in serum; its role in serum is uncertain.
b) IgD is primarily found on B cell surfaces where it functions as a receptor for antigen.
c) IgD does not bind complement.
Figure-11- showing the structure of Ig D
E. IgE
1. Structure
IgE exists as a monomer and has an extra domain in the constant region.
2. Properties
a) IgE is the least common serum Ig since it binds very tightly to Fc receptors on basophils and mast cells even before interacting with antigen.
b) Involved in allergic reactions – As a consequence of its binding to basophils and mast cells, IgE is involved in allergic reactions. Binding of the allergen to the IgE on the cells results in the release of various pharmacological mediators that result in allergic symptoms.
c) IgE also plays a role in parasitic helminth diseases. Since serum IgE levels rise in parasitic diseases, measuring IgE levels is helpful in diagnosing parasitic infections. Eosinophils have Fc receptors for IgE and binding of eosinophils to IgE-coated helminths results in killing of the parasite.
d) IgE does not fix complement.
Figure-12- showing the structure of Ig E
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