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Hybrid Antibodies
When antibody molecules are modified or designed using recombinant DNA technology to suit specific applications, such antibodies are called recombinant or hybrid antibodies, and the approach itself is termed as antibody engineering.

The essential steps in antibody engineering are:
(1) to develop the antibody design to serve the specific purpose,
(2) to bring together the DNA sequences that would generate this antibody molecule into a suitable expression vector,

(3) to introduce this recombinant DNA into a myeloma cell line where the recombinant antibody gene expresses itself,
(4) to mass culture the transfected (i.e., containing the recombinant antibody gene) clones of myeloma cell line, and
(5) to harvest the recombinant antibodies produced. It is essential that the myeloma cell line used for transfection is Ig- (immunoglobulin negative), i.e., does not produce any antibody of its own.

1. A recombinant antibody could be constructed so that the constant end of its heavy chain is fused to a polypeptide chain having an enzymatic function. The enzymatic polypeptide chain could replace the CH2 and CH3 regions of the heavy chain and be fused to the CH1 segment.
Such an antibody will be extremely useful for ELISA as there will be no need for a second antibody.
A gene producing such an antibody can be produced by fusing the heavy chain gene having the appropriate L-V-D-J and yl sequences (Appendix-4.1) with the sequence coding for the selected enzyme function.

2. The heavy chain gene of an antibody specific to a tumour-specific antigen may be fused with a gene encoding a toxin polypeptide. Such hybrid antibodies will carry the toxin specifically to the tumour cells and, thereby, kill them.

3. Gene segments encoding the variable region or V-region (involved in antigen-antibody interaction) of an antibody have been fused with the constant region or C-region of another antibody to yield a hybrid antibody.
A hybrid antibody has the antigen specificity of the first antibody (which contributed the V-region segments), but its other properties are due to the second antibody (contributing the C­ region segments).
This approach has been used to produce hybrid antibodies with their antigen-binding portion from a mouse antibody gene, and their C-region from the human antibody gene. Such antibodies are often known as humanized rodent antibodies. This is very useful in cases where it is not possible to either immunize humans or to induce antibody production in cultured human cells.

4. The knowledge of three dimensional structures of antigens and antibody regions involved in antigen binding can be used to predict the amino acid sequence of V -region of the antibody specific to the given antigen.

This involves a detailed computer modelling using the data on molecular organisation of the antigen to predict the corresponding organisation of the specific antibody. The gene encoding an antibody having the given amino acid sequence can now be synthesized and expressed in a suitable host cell.
The hybrid antibodies are monoclonal in nature since each preparation has antibodies of a single specificity.
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