6. Complement

Introduction to the Complement System

  • The Complement System consists of a collection of blood and approximately 30 cell surface proteins.

  • It plays a vital role in both innate and adaptive immune responses and is known for its antimicrobial functions.

  • Main Functions of the Complement System:

    • Protects the host from infection/inflammation.

    • Enhances phagocytosis of pathogens by recruiting innate immune cells.

    • Leads to the lysis (destruction) of target cells.

  • Mechanism of Action:

    • Functions as an enzyme cascade that clears pathogens.

    • Can bind to immunoglobulins (antibodies) and membrane components of cells.

    • Some complement proteins are pro-enzymes, which can cleave other complement components.

Key Features of the Complement System

  • Complement proteins are heat-labile, meaning they are easily destroyed by heat.

  • The complement system has five main functions:

    • Opsonization: Enhancing the ability of phagocytic cells to engulf pathogens.

    • Cell Lysis: Destroying pathogens by creating pores in their membranes.

    • Inflammation: Inducing inflammatory responses to recruit immune cells.

    • Chemotaxis: Attracting immune cells to the site of infection.

    • Immune Complex Clearance: Removing immune complexes formed by antigens and antibodies.

Characteristics of Complement Proteins

  • Found in fresh, normal serum and synthesized primarily by the liver.

  • Comprises 4% to 5% of total serum proteins and categorized as beta globulin.

  • Present in inactive precursor form; activation occurs upon stimulation by first complement component.

Activation Mechanisms of the Complement System

  • Complement can be activated through two main pathways:

    • Innate Immune Response: No antibodies or T cell receptors involved.

    • Adaptive Immune Response: More potent when IgG or IgM antibodies bind to antigens, exposing the Fc region that binds the first complement protein, C1.

Complement Components and Their Functions

  • The most abundant complement component is C3, which is non-specific and activated immediately in pathogen presence.

  • Components are identified by the letter C followed by their numerical designation, ranging from C1 to C9.

  • C3a and C3b: Fragments that play roles in inflammation and opsonization, respectively.

  • Complement Categories:

    1. Anaphylatoxins:

      • Induce redness, heat, and swelling; promote inflammation by binding to receptors on mast cells.

      • Trigger histamine release, affecting vascular permeability and smooth muscle contraction.

    2. Chemotactic Factors:

      • Initiate healing processes by attracting neutrophils and other repair cells.

    3. Opsonins:

      • Facilitate phagocytosis by binding foreign materials and enhancing recognition by phagocytic cells.

Detailed Components of the Complement System

  • Basic Components:

    • C1: subcomponents include C1q, C1r, C1s

    • C2

    • C3: critical component of the complement cascade

    • C4

    • C5

    • C6

    • C7

    • C8

    • C9

Specimen Requirements for Complement Activation

  • Blood samples must be properly stored to maintain complement activity.

  • Recommended storage conditions:

    • 14 days at 4° C: >60% complement remains

    • 2 months at -20° C: >60% complement remains

    • 3 months at -55° C: >60% complement remains

    • 24 hours at 37° C: <30% complement remains

    • 48 hours at room temperature: <40% complement remains

    • 72 hours at room temperature: 0% complement remains

Complement Activation Pathways

  • Three main pathways for complement activation:

    1. Classical Complement Pathway: Initiated by antigen-antibody complexes.

    2. Alternative Complement Pathway: Does not require antibodies; activated by pathogens directly.

    3. Mannose-Binding Lectin (MBL) Pathway: Activated by lectins binding mannose on pathogen surfaces.

Classical Pathway Details

  • Activation Phase (Recognition Phase):

    • Antibody (IgM or IgG) binds to antigens.

    • C1 complex (C1q, C1r, C1s) binds to the antibody-antigen complex in the presence of ionized calcium (Ca++).

  • C1q induces conformational change activating C1r and C1s, where C1s cleaves C4 into C4a and C4b.

  • C4b attaches to the target cell and acts on C2.

  • This forms C4b2a, known as C3 Convertase, crucial for the cascade.

Amplification Phase

  • In the presence of C4b2a, C3 is cleaved into C3a and C3b.

  • C3b binds to C4b2a to become C4b2a3b, referred to as C5 Convertase.

Membrane Attack Phase

  • Components C5, C6, C7, C8, and C9 are involved, forming the Membrane Attack Complex (MAC).

  • C5b binds to target cell surfaces and recruits C6, C7, C8, and multiple C9 monomers, modifying the target cell membrane and creating pores.

  • This causes osmotic lysis of the cell, resulting in hemolysis and necrosis.

Complement and Blood Transfusion

  • At the transfusion, compatibilities between patient plasma and donor red cells are crucial to prevent hemolysis.

  • Types of Hemolysis:

    • Intravascular Hemolysis: Destruction occurs within blood vessels.

    • Extravascular Hemolysis: Destruction occurs outside the vascular system, commonly in the liver or spleen.

Diagnostic Tests Related to Hemolytic Reactions

  • Direct Antiglobulin Test (DAT): Determines if red blood cells are sensitized in vivo with immunoglobulin or complement, used for various hemolytic conditions.

  • Indirect Antiglobulin Test (IAT): Detects unexpected antibodies in the plasma; assesses compatibility prior to transfusion to prevent reactions.

Conclusion

  • Understanding the complement system is essential for safe blood transfusion practices as it plays a central role in mediating immune responses and potential transfusion reactions.

  • Transfusion science must account for the complement activation pathway to predict and manage hemolytic reactions effectively within patients receiving transfusions.