The present invention relates to an antibody expression library derived from a patient which has been immunochallenged with one or more foreign antigens associated with a particular disease or foreign agent, wherein said patients have been immunochallenged with the foreign antigens at a time point such that they still contain a repertoire of antibody producing cells which are enriched with cells producing antibodies directed to said foreign antigens associated with said disease or foreign agent, or at a time point such that they are still in an active phase of immune response to said foreign antigens associated with said disease or foreign agent. Methods of producing such expression libraries are also disclosed.