The 1D6 monoclonal antibody specifically binds to CD81 which is also known as, Target of the antiproliferative antibody 1 (TAPA1 or TAPA-1), Tetraspanin-28 (Tspan-28 or TSPAN28), or Common variable immunodeficiency 6 (CVID6). This ~26 kDa surface protein is encoded by CD81 that belongs to the tetraspanin or transmembrane 4 (TM4SF) superfamily. CD81 is composed of four transmembrane and two extracellular domains with intracellular N- and C-termini. CD81 has a broad cellular distribution. It is variably expressed on thymocytes, T lymphocytes, B cells, NK cells, eosinophils, germinal center follicular dendritic cells, monocytes, endothelial cells, epithelial cells, fibroblasts, and at low levels on neutrophils. CD81 is involved in signal transduction and the regulation of cellular growth and adhesion. It serves as a component of a multimolecular complex comprised of CD19, CD21, and CD225 that regulates the activation and growth of B cells. Abnormal CD81 expression is implicated in the growth regulation of some lymphomas and in CVID6 immunodeficiency which is characterized by low antibody production. CD81 can also serve as a receptor for hepatitis C virus (HCV). The 1D6 antibody can reportedly induce homotypic adhesion amongst hematolymphoid cells, inhibit the proliferation of some B-cell lines, and costimulate anti-CD3 antibody activated T cells.