9 Ig Classes: Structure and Function

Dr. M. N. Gupta

 

  1. Objectives
  •  To learn how the structure of IgG was established
  •  Different classes of Ig
  •  Their structures and functions
  1. Concept Map
  1. Description

We referred to humoral immunity earlier. Antibodies are present in the serum and these proteins were originally detected there.

 

The early electrophoretic studies of the serum revealed the presence of several proteins with their characteristic migration patterns under the influence of the electric field.

 

The terms antibodies and immunoglobulins are often used interchangeably.

 

These proteins are produced by the immune system in response to the exposure to antigens.

 

As all animals are invariably exposed to antigens, some antibodies are always present in the serum.

 

Any intentional or accidental exposure to specific antigens produces the corresponding specific antibody population in the serum.

The first break through came from looking at a homogeneous protein obtained from tumours of plasma cells

In 1847, Henry Bence Jones, a physician at St. George hospital in London published a paper “On a New Substance Occuring in the Urine of a Patient With Mollities Ossium, Phil. Tr. Royal Soc. London. 138:55-62”

 

“On the 1st of November 1845, I received from Dr. Watson the following note, with a test tube containing a thick, yellow, semi-solid substance: “The tube contains urine of very high specific gravity; when boiled it becomes highly opaque; on the addition of nitric acid it effervesces, assumes a reddish hue, becomes quite clear, but, as it cools, assumes the consistence and appearance which you see: heat reliquifies it. What is it?””

 

Thus  A Bence Jones protein was discovered which is a monoclonal globulin protein or immunoglobulin light chain found in the urine, with a molecular weight of 22-24 kDa. Detection of Bence Jones protein may be suggestive of multiple myeloma.

 

Mollitis Ossimum is now called multiple myeloma, the cancer of the plasma cells. These Bence Jones proteins (despite 700 papers!) remained a biochemical curiosity but nevertheless these were valuable diagnostic test for multiple myeloma.

As we have discussed, any antigen would have a number of antigenic determinants. Hence the antibodies produced in response to the exposure to an antigen is bound to produce a population of immunoglobulins.

 

All immunoglobulins are specific to that particular antigen but each in fact is specific to a particular antigenic determinant.

For much of the work in immunology, especially in the early days, the serum as such constituted the antibody preparation. When used as such, it is, for obvious reasons, often referred to as antiserum.

 

In humans, there are five classes of antibodies or immunoglobulins.

 

The different types of immunoglobulins are of different shapes.

 

Immunoglobulin G (IgG)

 

This is responsible for the major part of the immunity against blood borne infectious agents.

 

It has the highest concentration of 8-16 mg/mL in normal adult serum as compared to all other classes of the immunoglobulins.

 

It has four different sub classes:

 

IgG1, IgG2, IgG3, IgG4

 

These different classes differ slightly in the amino acid sequences in a particular region called constant region in the H-chain (We will look at the different polypeptide chains of immunoglobulins shortly). These differences are reflected in their functional activities.

 

Ig is the only immunoglobulin which can cross the placental barrier and thus provides passive humoral immunity to the developing foetus.

IgG Structure

 

All IgG immunoglobulins have a four chain structure. This 4 chain structure of IgG is in fact the basic structural unit present in all classes of the immunoglobulins

Antibodies have 5 different kinds of heavy chains called µ, δ, γ, ε and α chains. These H-chains are present in IgM, IgD, IgG, IgE and IgA respectively and distinguish these different classes of antibodies/immunoglobulins.

 

In the subclases of IgG the heavy chains are different and are called γ1, γ2, γ3, γ4 and belong to the respective sub classes.

 

The molecular weights of the H-chain in IgG is in the range of 50-55 kDa. In any given immunoglobulin molecule, the two heavy chains and the two light chains are identical, giving an antibody molecule two identical antigen-binding sites, and thus the ability to bind simultaneously to two identical structures.

 

Immunoglobulins have two different kinds of light chains. These two L-chains are called κ (kappa) and λ (lambda). Each has a molecular weight of 23 kDa. A given immunoglobulin either has κ chains or λ chains, never one of each. No functional difference has been found between antibodies having λ or κ light chains, and either type of light chain may be found in antibodies of any of the five major classes.

 

The two heavy chains are linked to each other by disulfide bonds and each heavy chain is linked to a light chain by a disulfide bond.

 

These intramolecular disulphide bridges create compact structural units called domains. These domains are shown in the Figure below as VH, VL, CH1, CH2, CH3. The domains are also sometimes called as regions.

 

The IgG molecule consists of two symmetrical units joined by one interchain disulphide bridge.

Fab and Fc fragments

 

The N-terminal half of the H-chain and all of the L-chain are also connected with an interchain disulphide bridge. This part of the molecule is called Fab fragment. The subscript ab refers to the fact that this portion has the binding site for the antigen. Thus „ab‟ subscript refers to antigen binding property of the fragment F. As the immunoglobulin molecule is symmetrical, each IgG has 2 Fab fragments.

The Fc Fragment

 

The three quarter of the H-chains from the C-terminal side and the C-terminal half of the L-chains are constant. All antibodies of the same class and sub class have the same amino acid sequence. Hence these regions have been denoted as CH with C referring to constant. The C-terminal half of each H-chain constitutes Fc fragments. The subscript „c‟ refers to the fact that it was found to be easily crystallizable for X-ray diffraction studies. Thus Fc stands for fragment crystallized.

 

Fc fragment is not involved in the antigen binding but binds to complement and some other cells of the immune system. This will be discussed in the next lecture.

 

There is a hinge region around the place where Fab fragments join the Fc fragment. This is more flexible region and provides the required flexibility to the two Fab fragments to move with respect to each other (Figure 8).

 

R.R.Porter at Oxford, UK used a protease papain to break IgG into three fragments of nearly equal size.

Further work by Michael Green and Robert Valentine showed that another protease pepsin cleaved Ig differently.

Hypervariable regions

 

The following Kabat and Wu plot shows the variability of the amino acid sequences in both H-chains and L-chains among different antibodies. The different antibodies are against different antigens/antigenic determinants. Hence the variability is the origin of the antibody specificity. Hypervariable regions constitute the antigen binding sites. Affinity labeling confirmed the existence of hypervariable regions. Reactive haptens were found to react to the antigen binding sites (the hypervariable regions). Hypervariable regions are called complementarity determining regions (CDRs).

 

The five classes of Ig differ in the nature of their H-chain, chain structure and some other structural features. The basic structural unit is similar not only among the five classes of Ig but is also characteristic of other molecules associated with the immune system. Hence, from the evolutionary perspective, many of these arose from an ancestral gene. All these molecules, including the five classes of Ig are thus part of the super family.

 

IgG is found in the vascular spaces, extravascular spaces and secretions. Major IgG is found in the serum as well as during the secondary response to many antigens. In humans, IgG is capable of moving across placenta and thus protect the growing foetus. In humans, IgG has four subclasses as mentioned before. There concentrations are different in human serum with IgG1 having the highest concentration. All can cross the placental barrier but differ in the nature of their heavy chains. All IgG subclasses except IgG4 can bind to C1q to activate the classical pathway. It can act as opsonin. Fc part plays an important role. For example, Many large granular lymphocyte have Fc receptor.

 

FcR4 is the receptor on the placental cells that can bind to all Ig subclasses and enable their foetal circulation.

 

IgM

 

IgM is a pentamer of the 4 chain structure. During development of the immune system, IgM is the first Ig to be produced. It is also the first Ig to be produced during the primary immune response. It is capable of complement fixation. T-independent antigens produce antibodies with IgM as the main component. It has a molecular weight of 970 kDa, making it the largest Ig. Its heavy chain is µ. It has J-chain which is also synthesized by B-cells but coded by a gene different from those coding for Ig.

IgD is present in serum in traces. It is co-expressed alongwith IgM as a cell surface receptor on differentiating B-cells. Mature B-cells (plasma cells) do not have IgD on their surfaces. During encounter of B-cells with antigens, IgD regulates B-cell function leading to internalization of the antigen which after processing is presented to Tn cells.

It also has two subclasses IgA1 and IgA2 with identical molecular weight of 160 kDa. The subclasses have α1 and α2 as H-chains. The concentration of IgA1 in human serum is much higher than IgA2. IgA protects the mucous membranes and is present in all secretions as the most abundant Ig. In those specie in which IgG is not transferred across the placenta, It is IgA which protects neonates. It is present in colostrums. IgA can form dimmers and polymers. It is present in humans as monomers only and mostly as dimmers in other animals. It contains a secretory component SC and a joining chain (J-chain). SC stabilizes IgA and is involved in its transport of exocrine IgA. J-chain helps in the dimerization of secretory IgA (dimeric).

 

IgE

Its serum concentration is lowest among all the Ig under normal conditions. Its heavy chain is ε and molecular weight is 188 kDa. Apart from IgM, it is the only Ig having 5 heavy chain domains. Other Ig have 4 heavy chain domains. It is not involved in complement activation. IgE binds to mast cells and basophils which have high affinity Fc receptors. Consequently, these cells release mediators of inflammation after contact with the antigen. IgE mediates type I hypersensitivity reactions such as asthma and hay fever. It plays a very important role in protection against helminthic infections.

Summary:

 

  •  Bence Jones proteins (light chains) turned out to be the first step in understanding IgG structure
  •  IgG sequence was established first and it has two H-chains and 2 L-chains
  •  IgG structure is the basic structural unit for the structure of other clases of Ig
  •  There are five classes of Ig: IgG, IgA, IgM, IgD, IgE
  •  Their structures are now known and their individual functions is also now fairly well understood.