Recombinant proteins are those proteins that are produced in a species or cell line different from the original one.
This is possible thanks to genetic engineering, whereby the DNA sequence coding for the protein of interest is inserted into a bacterial plasmid (extra-chromosomal DNA structure of autonomous replication and easy transmission), which is transfected into a host cell capable of 1) replicating it creating a high number of copies, 2) transcribing it into messenger RNA and 3) translating it into a sequence of amino acids that will take on different structures until a functional protein is obtained.