C1q

The first component of a complement is a pentameric complex comprising of C1q and the Ca²⁺-dependent tetramer C1r2C1s2. The roles of C1q within this complex are to recognise and bind immune complexes, as well as pathogen- or danger-associated molecular ligands, and trigger activation of the classical pathway of complement. However, data accumulated to date has also shown that C1q is not limited to its traditional role but also plays a versatile role in a growing list of physiologic and pathologic processes that include chemotaxis, angiogenesis, infection, inflammation and cancer. Although complement proteins in general are synthesised in the liver, C1q is unique in that, cells of myeloid origin, as well as proliferating cells and tumour cells synthesise it. Locally secreted C1q functions in an autocrine/paracrine manner to induce proinflammatory and anti inflammatory cytokines and as such may be regarded as a 'complekine' - a complement protein with cytokine-like activity.