The immune system uses a sophisticated profiling method to identify and subdue foreign pathogens. Using small protein fragments from viruses or bacteria, antigen-presenting cells (APCs) prime armies of lymphocytes to look for matching traits elsewhere in the body. Whether APCs find such proteins within their cytoplasm or phagocytose them from outside usually determines which processing pathway is used and what kind of lymphocytes are called to service. But sometimes antigens switch tracks, moving from endocytic compartments to the cytosolic pathway. This is termed cross-presentation, and how proteins breach cellular barriers to access the inside track has been a big question in immunology.