Complement System

The complement system is a complex series of proteins that work together to identify and destroy pathogens. It is an important part of the innate immune response, as it can rapidly respond to a wide range of pathogens without the need for prior exposure.

There are three main pathways by which the complement system can be activated:

  1. The classical pathway: This pathway is activated when antibodies bind to a pathogen, forming an antigen-antibody complex. The complex then binds to complement protein C1q, which initiates a series of cleavage reactions that lead to the activation of the complement system.
  2. The alternative pathway: This pathway is activated by the presence of certain molecules on the surface of pathogens. These molecules can directly activate complement protein C3, which initiates a series of cleavage reactions that lead to the activation of the complement system.
  3. The lectin pathway: This pathway is activated when molecules called lectins bind to the surface of pathogens. The lectins then bind to complement protein C2, which initiates a series of cleavage reactions that lead to the activation of the complement system.

Once the complement system is activated, it can lead to a variety of immune responses, including:

  1. Opsonization: Complement proteins can bind to the surface of pathogens, making them more recognizable and easier for phagocytic cells to engulf and destroy.
  2. Inflammation: Complement proteins can cause the release of inflammatory mediators, leading to the recruitment of immune cells to the site of infection.
  3. Membrane attack complex (MAC) formation: Complement proteins can form a complex called the MAC, which creates a pore in the membrane of the pathogen, causing it to lyse and die.

The complement system is tightly regulated to prevent excessive activation, which can lead to tissue damage and inflammation. Deficiencies in the complement system can lead to increased susceptibility to infections and other immune disorders, while overactivation of the complement system can lead to autoimmune diseases and other inflammatory disorders.