National Institutes of Health (NIH) scientists have identified a protein that plays matchmaker between two key types of white blood cells, T and B cells, enabling them to interact in a way that is crucial to establishing long-lasting immunity after an infection. Their finding may also explain why some individuals who have a genetic defect that prevents them from making this protein—called SAP—suffer from lethal infections with a common virus that otherwise is rarely fatal (Epstein-Barr virus), while others with this genetic defect have problems with B-cell lymphomas.