The Complement System

Introduction to the Complement System

The complement system consists of a set of proteins that interact with one another and are critical to the body's immune response. These proteins are produced predominantly in the liver and released into the bloodstream. The system is named 'complement' because it complements the action of antibodies in eliminating pathogens. It can be activated through various stimuli, and its components act in a cascade, amplifying the immune response.

History and Recognition

  • The complement system was discovered in the 1890s and was identified as a heat-labile substance in normal nonimmune serum.

  • Paul Ehrlich named it the 'complement' for its role in enhancing antibody action against microorganisms.

  • Jules Bordet received the Nobel Prize in 1919 for elucidating the nature of complement.

Properties of the Complement System

  1. Presence Across Species: The complement system is found in the serum of all mammals and various lower animals, including birds and fishes.

  2. Heat-Lability: Complement proteins can be inactivated by heating serum to 56°C for 30 minutes.

  3. Complex Composition: It consists of more than 30 soluble and membrane-bound proteins.

  4. Production Sites: Mainly produced by hepatocytes, as well as monocytes and epithelial cells in various tracts.

  5. Binding Characteristics: Typically binds to antigen-antibody complexes rather than free antigens.

  6. Roles in Immunity: Participates in both innate and adaptive immune responses.

  7. Synthetic Rates: Production of complement proteins increases during infections.

  8. Cascade Reactions: Initiated by an enzyme cascade that leads to numerous biological effects.

The Cascade System

  • The complement system operates as a cascade where one reaction initiates subsequent reactions. This leads to an exponentially growing response, enhancing the effectiveness of the immune defense.

Components of the Complement System

  • The complement components are designated by numbers (C1-C9), letters (e.g., factor D), or trivial names.

  • Cleavage fragments are denoted with small letters, where the smaller fragment is usually marked as 'a' and the larger as 'b' (except for C2).

  • Larger fragments interact locally while smaller fragments can diffuse, triggering inflammatory responses.

Functions of the Complement System

  • Cell Lysis: Kills cells, bacteria, and viruses.

  • Opsonization: Enhances phagocytosis of antigens.

  • Receptor Binding: Activation of specific immune system functions via receptors.

  • Immune Clearance: Removal of immune complexes from circulation to organs like the spleen and liver.

Activation Pathways of Complement

  1. Classical Pathway: Triggered by antigen-antibody complexes.

  2. Alternative Pathway: Antibody-independent activation, initiated by C3b binding to pathogens and involves properdin, factor B, and factor D.

  3. Lectin Pathway: Also antibody-independent, initiated by mannose-binding lectin binding to microbial surfaces, similar to the classical pathway in subsequent actions.

Classical Pathway Steps

  • Begins with C1 binding to an antigen-antibody complex.

  • Activation of C1q leads to cascading reactions activating C3 convertase and subsequently C5 convertase, culminating in the formation of the Membrane Attack Complex (MAC).

Alternative Pathway Steps

  • Involves spontaneous hydrolysis of C3.

  • C3b binds to foreign substances, attaches to factor B, and factor D activation leads to the formation of C3bBb, which acts as a C3 convertase, amplifying the complement response.

Lectin Pathway Steps

  • Activated by mannose-binding lectin (MBL) binding to microbial surfaces, resulting in the activation of C4 and C2, leading to C5 convertase creation, similar to the classical pathway.

Membrane Attack Complex (MAC)

  • The terminal component of complement activation, involving C5b, C6, C7, C8, and C9, which creates a pore in target cell membranes leading to cell lysis.

Regulation of the Complement System

  • Due to its potential to damage host tissues, the complement system is tightly regulated.

  • Regulatory Proteins: Proteins such as C1 Inhibitor, C4b-binding protein, and complement receptor type 1 are critical in preventing unwanted complement activation.

  • Mechanisms: These proteins inhibit complement activity before assembly of convertases or during the formation of MAC.

Biological Effects of Complement Activation

  • Activation can lead to cell lysis, promote an inflammatory response, facilitate opsonization, and enhance clearance of immune complexes.

  • Key complement products like C3a, C4a, and C5a (anaphylatoxins), play a vital role in inflammation and immune cell recruitment.

Conclusion

The complement system is a crucial component of the immune response, enhancing both innate and adaptive immunity. Its regulation is essential to prevent tissue damage, emphasizing the need for tight control over its activation. Understanding the complement pathways provides insight into potential immunological therapies and treatments for complement-related diseases such as paroxysmal nocturnal hemoglobinuria.