Complement System

Definition

The complement system is a complex network of proteins that work together as part of the innate immune response to help eliminate pathogens and damaged cells from the body. It acts as a complement to the antibody-mediated adaptive immune response, hence the name 'complement system'.

5 Must Know Facts For Your Next Test

1. The complement system is composed of over 30 different proteins that work in a cascading series of enzymatic reactions to eliminate threats.
2. Activation of the complement system can occur through three distinct pathways: the classical pathway, the alternative pathway, and the lectin pathway.
3. Complement proteins can bind to and tag pathogens, marking them for destruction by phagocytic cells in a process called opsonization.
4. The membrane attack complex (MAC) is a structure formed by the complement system that punches holes in the membranes of target cells, causing them to lyse and die.
5. Anaphylatoxins, such as C3a and C5a, are small complement proteins that act as chemical messengers, triggering inflammation and attracting phagocytic cells to the site of infection or damage.

Review Questions

• Explain the role of the complement system in the innate immune response, as described in Section 21.2 Barrier Defenses and the Innate Immune Response.
  • The complement system is a crucial component of the innate immune response, as described in Section 21.2. It acts as a first line of defense against pathogens by directly attacking and lysing them through the formation of the membrane attack complex. Additionally, the complement system facilitates the process of opsonization, marking pathogens for phagocytosis by immune cells. The release of anaphylatoxins also helps to recruit and activate other innate immune cells, such as phagocytes, to the site of infection or damage.
• Describe how the activation of the complement system contributes to the immune response against pathogens, as discussed in Section 21.5 The Immune Response against Pathogens.
  • As outlined in Section 21.5, the activation of the complement system plays a significant role in the immune response against pathogens. The three distinct pathways of complement activation (classical, alternative, and lectin) can be triggered by the presence of foreign microorganisms, leading to the opsonization of pathogens and the formation of the membrane attack complex. This helps to directly eliminate the threat and also facilitates the recruitment and activation of other immune cells, such as phagocytes, to further enhance the body's defense against the invading pathogens.
• Analyze the relationship between the complement system and the lymphatic and immune systems, as described in Section 21.1 Anatomy of the Lymphatic and Immune Systems.
  • As discussed in Section 21.1, the complement system is closely integrated with the lymphatic and immune systems. The complement system acts as a bridge between the innate and adaptive immune responses, complementing the work of antibodies and other adaptive immune components. The activation of the complement system can directly lyse pathogens, but it also plays a crucial role in opsonization, which enhances the ability of phagocytic cells to recognize and destroy foreign organisms. Furthermore, the release of anaphylatoxins by the complement system helps to recruit and activate other immune cells, such as those found in the lymphatic system, to mount a comprehensive and coordinated response against threats to the body.

Related terms

Opsonization:

The process by which complement proteins bind to the surface of a pathogen, marking it for phagocytosis and destruction by immune cells.

Membrane Attack Complex (MAC): A structure formed by the complement system that punches holes in the membranes of target cells, causing them to lyse and die.

Anaphylatoxins: Small complement proteins that act as chemical messengers, triggering inflammation and attracting phagocytic cells to the site of infection or damage.