CD81 antigen is a ~25 kDa protein. Structural component of specialized membrane microdomains known as tetraspanin-enriched microdomains (TERMs), which act as platforms for receptor clustering and signaling. Essential for trafficking and compartmentalization of CD19 receptor on the surface of activated B cells. Upon initial encounter with microbial pathogens, enables the assembly of CD19-CR2/CD21 and B cell receptor (BCR) complexes at signaling TERMs, lowering the threshold dose of antigen required to trigger B cell clonal expansion and antibody production. In T cells, facilitates the localization of CD247/CD3 zeta at antigen-induced synapses with B cells, providing for costimulation and polarization toward T helper type 2 phenotype. Present in MHC class II compartments, may also play a role in antigen presentation. 1. Shoham, T. et al. (2006) Molecular and cellular biology 26, 1373-85. PMID: 164496492. van Zelm, MC. et al. (2010) The Journal of clinical investigation 120, 1265-74. PMID: 202374083. Zimmerman, B. et al. (2016) Cell 167, 1041-1051.e11. PMID: 278813024. Cherukuri, A. et al. (2004) The Journal of biological chemistry 279, 31973-82. PMID: 151619115. Sagi, Y. et al. (2012) Proceedings of the National Academy of Sciences of the United States of America 109, 1613-8. PMID: 223076196. Rocha-Perugini, V. et al. (2013) Molecular and cellular biology 33, 3644-58. PMID: 238580577. Secrist, H. et al. (1996) European journal of immunology 26, 1435-42. PMID: 87665448. Schick, MR. et al. (1993) Journal of immunology (Baltimore, Md. : 1950) 151, 4090-7. PMID: 84093889. Takeda, Y. et al. (2003) The Journal of cell biology 161, 945-56. PMID: 12796480 | 
