Complement

Overview of the Complement System

The complement system is a complex network of over 50 proteins that work together as part of the innate immune system. It plays a vital role in defending the body against pathogens and clearing cellular debris. Activation of the complement cascade leads to a series of amplified biological responses.

Key Concepts: Complement Activation Pathways

There are three primary pathways for complement activation:

• Classical Pathway: Initiated by antibody binding to antigen on a pathogen’s surface.
• Lectin Pathway: Triggered by mannose-binding lectin (MBL) binding to microbial carbohydrates.
• Alternative Pathway: Spontaneously initiated on microbial surfaces or by certain host molecules.

Deep Dive: Mechanisms of Action

Once activated, the complement cascade results in several effector functions:

• Opsonization: Complement proteins coat pathogens, marking them for phagocytosis.
• Inflammation: Release of anaphylatoxins (C3a, C5a) recruits immune cells and promotes inflammation.
• Cell Lysis: Formation of the Membrane Attack Complex (MAC) creates pores in pathogen membranes, leading to lysis.

Applications and Clinical Significance

The complement system is essential for fighting bacterial infections, particularly those caused by encapsulated bacteria. Deficiencies in complement proteins can lead to recurrent infections and autoimmune diseases. Therapeutic strategies aim to modulate complement activity in conditions like autoimmune disorders and transplantation.

Challenges and Misconceptions

A common misconception is that complement is solely an antibody-dependent system. However, the lectin and alternative pathways highlight its innate immune role independent of adaptive immunity. Regulating this powerful system is crucial to prevent self-damage.

Frequently Asked Questions

What is the main role of complement?

Its main role is to help clear pathogens and damaged cells from the body, working alongside other immune components.

How is complement regulated?

Regulation occurs through specific inhibitory proteins that prevent uncontrolled activation and damage to host tissues.