What expression systems are commonly used for recombinant antibody libraries?

Recombinant antibody libraries require highly competent (easy to transfect) expression systems that are capable of expressing, processing, and secreting the recombinant antibody. 

These are the expression systems commonly used for recombinant antibody libraries:  

• Bacteria (e.g., Escherichia coli), yeast or insects: Bacteria (E.coli), yeast or insects are favored for expressing recombinant antibodies because of their ease of manipulation and rapid growth. However, these three expression systems are inefficient when it comes to properly folding and assembling complex proteins, resulting in low yield and suboptimal product quality.
• Mammalian Cell Lines (e.g., CHO cells or HEK293 cells): Mammalian cell lines, particularly Chinese Hamster Ovary (CHO) cells or Human Embryonic Kidney (HEK293) cells, are the preferred expression system for producing recombinant antibodies. HEK293 cells offer several advantages that make the preferred hosts for protein expression. They are simple to propagate and sustain, easy to handle, offer high protein yield and quick transfection rate, and are compatible with various transfection methods. However, because of limitations such as clumping tendencies, their use is limited to research settings. 
• CHO Cells (Chinese Hamster Ovary Cells): CHO cells adapt well to serum-free media, exhibit robust growth in suspension cultures, and can produce recombinant antibodies at a large scale with high specificity. Originating from hamsters, these cells are less susceptible to human viral contaminants while retaining the ability to perform glycosylation compatible with humans.