When a target antigen is an integral or transmembrane protein, the antigen often requires the cell membrane to be properly folded for presentation to the immune system.This is especially true for GPCRs and ion channels. Having the antigen in native conformation may be the difference between obtaining blocking/activating, flow cytometry, immunoprecipitating antibodies or just western blotting antibodies. When native cells are difficult to isolate in sufficient numbers, expressing the antigen in an easily grown cell lines may be the way to go.
Transfection is the process of deliberately introducing nucleic acids into eucaryotic cells. It may also refer to other methods and cell types: "transformation" is more often used to describe non-viral DNA transfer in bacteria, non-animal eukaryotic cells or plant cells. In animal cells, transfection is the preferred term as transformation is also used to refer to progression to a cancerous state (carcinogenesis) in these cells. Transduction is often used to describe virus-mediated DNA transfer.
When immunizing with transfected cells, it is important to make sure the antigen is expressed on the cell surface in the correct conformation. This can be tested y making sure it still possesses whatever biological function it showed in the native cell line. Sometimes a surface marker (tag) is incorporated in the transfecting plasmid so that the tag is not expressed unless the antigen has been transcribed. Caution should be exercised when using tags to be sure they do not interfere with folding of the antigen or block an epitope of interest. Screening fusions may be a little more challenging due to reactivity with other cell surface proteins but these are not insurmountable.