Complement System

Definition

The complement system is a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear pathogens from an organism. It consists of a series of proteins that work together to identify and eliminate foreign invaders, such as bacteria and viruses, and plays a crucial role in both innate and adaptive immunity.

5 Must Know Facts For Your Next Test

1. The complement system can be activated through three main pathways: the classical pathway, the lectin pathway, and the alternative pathway, each responding to different types of pathogens.
2. One important function of the complement system is forming membrane attack complexes (MACs), which create pores in the membranes of pathogens, leading to their lysis.
3. Complement proteins circulate in the bloodstream in an inactive form and must be cleaved into active fragments to exert their functions.
4. The complement system not only helps in pathogen clearance but also plays a role in bridging the innate and adaptive immune responses by enhancing the action of antibodies.
5. Dysregulation or deficiency in components of the complement system can lead to autoimmune diseases or increased susceptibility to infections.

Review Questions

• How does the complement system enhance the immune response against infectious agents?
  • The complement system enhances the immune response by promoting opsonization, where complement proteins coat pathogens, making them easier targets for phagocytes. It also activates inflammation through anaphylatoxins, attracting immune cells to the site of infection. Additionally, it forms membrane attack complexes that can directly lyse bacterial cells, providing a robust defense against infections.
• Compare and contrast the three activation pathways of the complement system and their roles in immune responses.
  • The classical pathway is initiated by antibody-antigen complexes, while the lectin pathway is triggered by specific sugars on pathogen surfaces. The alternative pathway can be activated directly on pathogen surfaces without antibodies. All three pathways lead to a common endpoint of complement activation, but they differ in how they recognize pathogens. This allows for a versatile response tailored to various types of infectious agents.
• Evaluate the implications of complement system deficiencies on human health and disease susceptibility.
  • Deficiencies in the complement system can significantly impact human health by increasing vulnerability to infections, particularly with encapsulated bacteria. Individuals with such deficiencies may also develop autoimmune disorders due to unchecked inflammation. Furthermore, these deficiencies can disrupt the balance between innate and adaptive immunity, leading to complex health issues that require careful management and targeted therapies to prevent infections and maintain immune function.

Related terms

Cascades: The sequential activation of complement proteins in response to a pathogen, leading to a rapid amplification of the immune response.

Opsonization: The process by which complement proteins coat pathogens, making them easier for phagocytic cells to recognize and engulf.

Anaphylatoxins: Small peptides produced during complement activation that promote inflammation and recruit immune cells to the site of infection.