complementtijärjestelmän

The complement system is a crucial part of the innate immune system. It is a complex cascade of plasma proteins that can be activated by pathogens or by antibodies bound to pathogens. Once activated, the complement system can lead to a variety of immune responses, including the lysis of target cells, opsonization of pathogens to enhance phagocytosis, and the recruitment of inflammatory cells.

There are three main pathways for complement activation: the classical pathway, the alternative pathway, and the lectin pathway. The classical pathway is typically initiated by antibodies binding to antigens on the surface of a pathogen. The alternative pathway can be activated directly by certain microbial surfaces in the absence of antibodies. The lectin pathway is triggered by the binding of mannose-binding lectin (MBL) or other lectins to carbohydrate structures on microbial surfaces.

Despite the different initiation mechanisms, all three pathways converge on the cleavage of C3, a central protein in the complement cascade. The resulting fragments of C3 have diverse functions. For example, C3b acts as an opsonin, coating pathogens and marking them for destruction by phagocytes. Another fragment, C5a, is a potent chemoattractant for neutrophils and other inflammatory cells.

The final stages of complement activation involve the formation of the membrane attack complex (MAC). MAC inserts itself into the cell membrane of the target pathogen, creating pores that lead to cell lysis. The complement system plays a vital role in host defense against bacterial and viral infections and is essential for maintaining homeostasis. Defects in complement components can lead to increased susceptibility to infections and autoimmune diseases.