CD81 (TAPA-1) is a tetraspanin protein broadly expressed on immune and endothelial cells, where it regulates cell signaling, adhesion, and migration. It is a critical receptor for hepatitis C virus (HCV) and malaria parasites, making it an important target in infectious disease research. In oncology, CD81 contributes to tumor progression, metastasis, and immune regulation, and is being explored as a therapeutic target. CD81 is also one of the most widely used exosome markers, essential for studying extracellular vesicle (EV) biology, biomarker discovery, and EV-based therapeutics. Catalina supports research in virology, immunology, oncology, and EV biology with CD81 antibodies that have passed stringent validation processes.