Classes of Antibodies

Figure 2 shows the most abundant class of antibodies found in blood serum and lymph - immunoglobulin G (IgG). IgG of molecular mass about 156 000, is most frequently used as a receptor in immunosensors. According to X-ray data6 8, IgG is a Y-shaped molecule consisting of two identical antigen binding Fab arms of dimensions 6.5 nm by 3.5 nm and an inactive Fc shank of dimensions 5 nm by 3.5 nm. 

The structure and classes of antibodies are described in many textbooks and as an introduction to immunology in method collections (16,17). 

Antibody classes In mammals, there are five classes of antibodies (also known as immunoglobulins IgG, IgA, IgM, IgE and IgD). Each immunoglobulin class differs in their heavy chain constant domains (see Fig. 1.2) and consequently in its biological properties. For immunoassays, two immuno-glubulin classes are of importance - IgG and IgM. 

Fc fragment (for Fragment, crystallizable) A region of an antibody (IgG molecule) composed of two heavy chains on the C-terminal side. Fc has many effector functions (e.g., binding complement, binding to cell receptors on macrophages and monocytes, etc.) and serves to distinguish one class of antibody from another. 

Whilst all immunoglobulins conform to the generalised pattern described above (i.e. each molecule comprises two L chains and two H chains), there are five different classes of antibodies that represent variations on this common theme. The L chains of these molecules may be cor A (classified according to their structure) the H chains are classified as a, 8, e,y or /x in IgA, IgD, IgG, IgE and IgM, respectively. Furthermore, some of these antibodies possess ancillary protein chains that form polymeric antibody structures. 

Polyclonal antibodies are produced by injecting an antigen into an animal in the presence of an adjuvant containing bacterial lipopolysaccharides that stimulate the immune system. Serum prepared from the blood contains several different classes of antibodies that interact with different domains in the antigen molecule, each of... 

Figure 17.14 Structure ofIgG, IgM and IgA classes of antibodies. Note that IgA can exist as a dimer or trimer.

ITABLE 10.1. Biological properties and localization of the five classes of antibodies... 

CYP cytochrome P450. cytolytic causing lysis of cells, cytoskeletal relating to the cytoskeleton. cytoskeleton network of protein fibrils within cells, cytotropic a class of antibodies that attach to tissue cells. 

There are five classes of antibodies or immunoglobulins.59 60 Tire first three, IgG, IgM, and IgA, are quantitatively the most significant, but IgD and IgE are also important. For example, the content of IgE is elevated in allergic persons. The basic structure of all of the immunoglobulins is that of a quasi-symmetric dimer composed of a pair of light chains and a pair of heavy chains whose lengths vary among the different... 

There are five different types of heavy chains, which correspond with five different classes of antibodies. The constant region of the heavy chain is identical in all antibodies of the same class. The classes of the immunoglobulin molecules differ based on the Fc region of the heavy chain, which are responsible for the different functions performed by each class. Thus the constant region confers on each class of antibody its effector function. 

After recognizing antigens through membrane-bound antibodies, there is B cell proliferation and differentiation for about 4-5 days. This results in the production of plasma and memory cells. One of the five classes of antibodies are produced and secreted by plasma cells that do not possess membrane-bound antibodies. Plasma cells survive for about 1-2 weeks. 

There is no one procedure or combination of procedures that is applicable to all cases. The class of antibody to be purified has to be considered. Conventional procedures for the production of polyclonal antisera will generally result in immunoglobulin G, and procedures for the generation of monoclonal antisera may result in any of the immunoglobulin classes. Immunoglobulins from different species have broadly similar properties, but protein A, for example, will not bind all of the subclasses of human IgG, nor is it particularly effective in the purification of IgG from rat, sheep, or goat (2). Thus a certain amount of trial and error may be required to reach the best protocol for a particularly demanding application, but the methods described here should provide an adequate purity for most purposes. 

The first examples of immunotherapy with semm emerged at the end of the nineteenth century, since which time different types of immunotherapy with various classes of antibodies have been developed that will, in the future, generate exciting new pharmaceuticals. To correctly classify and understand the therapeutic principle of antibodies, the constitution of the immune system of the human body will briefly be presented in the following sections. 

Five classes of Humans have five different classes of antibody molecule which differ both in immunoglobulins structure and in function. These are called immunoglobulin A (IgA), IgD, IgE, IgG and IgM and each has its own type of heavy chain a, 8, e, y and x, respectively. Thus IgA molecules have two identical a heavy chains, IgD molecules have two identical 8 heavy chains, etc. The human IgG class of antibodies is further divided into four IgG subclasses IgG, IgG2, IgG3 and IgG4, having y, y2, y3 and y4 heavy chains respectively. 

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