Brandtzaeg trained in microbiology and immunology at the Medical Center of the University of Alabama at Birmingham before earning his Ph.D. in immunology from the University of Oslo. Until recently he headed the Faculty Division of Rikshospitalet University Hospital, and he is the founder of its Laboratory for Immunohistochemistry and Immunopathology. Brandtzaeg's research examines the biology and pathology of the mucosal immune system, including the study of mucosal diseases associated with chronic inflammation, allergy, immunodeficiency, and malignant and reactive disorders of mucosal and peripheral lymphoid tissue. Brandtzaeg has been Norway's most cited researcher over the past two decades and was the first European president of the international Society for Mucosal Immunology.