Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. Antigen-Specific Monoclonal Antibodies Isolated from B Cells Expressing Constitutively Active STAT5 BACKGROUND: Fully human monoclonal antibodies directed against specific pathogens have a high therapeutic potential, but are difficult to generate. METHODOLOGY/PRINCIPAL FINDINGS: Memory B cells were immortalized by expressing an inducible active mutant of the transcription factor Signal Transducer and Activator of Transcription 5 (STAT5). Active STAT5 inhibits the differentiation of B cells while increasing their replicative life span. We obtained cloned B cell lines, which produced antibodies in the presence of interleukin 21 after turning off STAT5. We used this method to obtain monoclonal antibodies against the model antigen tetanus toxin. CONCLUSIONS/SIGNIFICANCE: Here we describe a novel and relatively simple method of immortalizing antigen-specific human B cells for isolation of human monoclonal antibodies. These results show that STAT5 overexpression can be employed to isolate antigen specific antibodies from human memory B cells. In general, vaccination is a safe and efficient way to protect the human body against specific pathogens. However, vaccination is only applicable as a preventive measure, and development of new vaccines is a slow and expensive process. As an alternative, the use of sera enriched for pathogen-specific antibodies has been suggested [1, 2] . Treatment with pathogen-specific antibodies could then be applied in a prophylactic as well as in a therapeutic setting. However, non-human derived sera often provoke an immune response, thereby limiting the maximum number of treatments. Other possible caveats are the fact that it is difficult to obtain large amounts of sera of the same quality, and the risk of contamination with pathogens, in particular with viruses such as but not limited to Human Immunodeficiency Virus (HIV) and Hepatitis C Virus (HCV).