The monoclonal antibody clone NKTA255 recognizes human leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1). The LAIR family consist of at least two members; LAIR-1 (CD305) and LAIR-2 (CD306). To date, several splice variants of LAIR-1 have been identified. LAIR-1 is an inhibitory receptor expressed on most immune cells, like NK cells, T- & B-cells, monocytes, DCs, megakaryocytes, eosinophils, basophils, and mast cells. Especially the expression on pDCs is high. The main ligand of LAIR proteins is collagen. LAIR-1 is a transmembrane receptor with a single extracellular Ig-like domain, a transmembrane region and a cytoplasmic tail. This tail consist of two immunoreceptor tyrosine-based inhibition motif (ITIM) domains, which are responsible for its immunomodulating action. After cross-linking, the tyrosines in the ITIMs become phosphorylated which is required for inhibition of cellular activation. By shedding a soluble variant of the protein arises. sLAIR-1 can be found in healthy individuals and can be used as a marker of lymphocyte activation. sLAIR-1 has been found among others in serum, urine, amniotic fluid, and synovial fluid. Besides collagen, a few other ligands are described like Complement C1q, MBL, and SP-D. Complement activation of the classical pathway is controlled by C1q. By binding of LAIR-1 to C1q, pathway initiation is hampered leading to diminished complement related phagocytosis and immune regulation.The protein has been associated with several (auto-) immune diseases. In rheumatoid arthritis expression is decreased on CD4+ T-cells and high in CD14+ monocytes synovial macrophages. Also in SLE lower levels of LAIR-1 has been found on pDCs. sLAIR-1 might be an indicator of the immune response after virus infection or organ transplants. LAIR-1 modulates apoptosis and cytokine secretion in THP-1 cells.