All antibodies are proteins known as gammaglobulins, or, more specifically, as immunoglobulins. There are 5 classes of immunoglobulins. IgG, IgM, IgA, IgD, and IgE. They differ according to their heavy chains.
The basic antibody molecule is a 4-chain molecule consisting of 2 heavy chains and 2 light chains held together by disulphide bonds, some of which are inter-chain and some of which are intra-chain (figure 2-7). Each part of the molecule has different functions:
The fab fragment: Fab stands for "fragment antigen binding" and is the part of the antibody that binds to antigens. This is also where the variable amino acid sequences occur.
The Fc fragment: Fc stands for "fragment crystallizable". The Fc fragment is where complement binds and is also where antihuman globulin serum (anti-IgG) will bind. The amino acid sequences here are constant.
NOTE: Papain will split antibodies into 2 Fab fragments and 1 Fc fragment. Thus the antibody can sensitize red cells but not agglutinate them.
A more complex description showing antibody domains is shown in Figure 2-8.
Comparison of Immunoglobulin Structures and Properties
Polyclonal/Monoclonal Antibodies
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