Complement Cascade Review

Introduction to the Complement Cascade

• Vital for Transfusion Scientists: Understanding the complement cascade is crucial for anyone working in transfusion science, despite its initial complexity.

• Innate Immune Defense: Complement is an integral part of our innate immune system, designed for rapid response to infections.

• Function: It helps tackle a variety of pathogens by causing cell lysis or flagging them for phagocytosis.

• Transfusion Context: In transfusion medicine, activation of the complement cascade due to a red cell antibody is an unintended consequence as transfusion of allogeneic red cells is an artificial situation.

Classical Complement Cascade: Activation Stage

• Initiation Event: The classical cascade begins when an antibody binds to its target antigen on the red cell surface.

• C1 Binding Requirement:

  • For IgG antibodies, at least two Fc components must be available to enable C1 binding. This means a minimum of two IgG antibodies must bind to very close proximity on their target antigens.

  • Spatial Arrangement: The location of red cell antigens can affect IgG's ability to activate complement; if antigens are spread out, two IgGs might not bind close enough for C1.

  • For IgM antibodies, a single bound IgM molecule has five available Fc portions, making it extremely efficient at C1 binding regardless of antigen spatial arrangement.

• Complement Protein Patrol: Complement proteins are continuously circulating, ready to act as needed.

• C1 Binding Details:

  • Requirement: Calcium ions $(Ca^{2+})$ are essential for C1 binding.

  • Specific Component: It is the $C1q$ component of the $C1$ protein that directly binds to the Fc portions of the bound antibodies.

  • Sequential Activation: This binding leads to the sequential activation of $C1R$ and $C1S$. The active complex is denoted as $C1qrs$.

• Cleavage of C4 and C2:

  • Activated $C1qrs$, bound to the antibody-antigen complex, cleaves $C4$ into $C4b$ and $C4a$.

  • Simultaneously, it cleaves $C2$ into $C2b$ and $C2a$.

  • Fate of Cleaved Components:

    • $C4a$ and $C2a$ enter the circulation and play no further role in the cascade at the red cell surface.

    • The remaining components, $C4b$ and $C2b$, form a complex known as C3 convertase $(C4b2b)$.

Classical Complement Cascade: Amplification Stage

• C3 Convertase Binding: The newly formed $C3$ convertase $(C4b2b)$ binds to the red cell membrane, usually close to the original antibody binding site.

• Cleavage of C3: Once bound, $C3$ convertase can cleave numerous passing $C3$ molecules.

  • $C3$ is cleaved into $C3b$ and $C3a$.

  • $C3a$: This short-lived fragment is an anaphylatoxin and a pro-inflammatory mediator. It detaches and enters circulation.

  • $C3b$: This component can either bind to the existing $C4b2b$ complex or directly to the red cell surface.

• Amplification: A single $C4b2b$ complex has the capacity to cleave hundreds of $C3$ proteins, which is why this stage is called the amplification stage.

• Formation of C5 Convertase: The binding of $C3b$ to the $C4b2b$ complex creates a new complex called C5 convertase $(C4b2b3b)$. This complex is capable of cleaving both $C3$ and $C5$ proteins in the vicinity.

Classical Complement Cascade: Membrane Attack Stage (MAC Formation)

• Initiation: The cascade transitions into the membrane attack stage once $C5$ convertase begins acting on $C5$ protein.

• Cleavage of C5:

  • $C5$ convertase cleaves $C5$ protein into $C5b$ and $C5a$.

  • $C5a$: Similar to $C3a$, $C5a$ is an anaphylatoxin, but it is a significantly more potent pro-inflammatory mediator. It also moves into circulation.

  • $C5b$: This component binds to the red cell membrane, initiating the formation of the Membrane Attack Complex (MAC).

• Sequential MAC Assembly:

  • Bound $C5b$ attracts $C6$.

  • $C5b6$ complex attracts $C7$.

  • $C5b67$ complex attracts $C8$.

  • $C5b678$ complex attracts multiple $C9$ proteins.

• Pore Formation: If $C9$ binding is not inhibited, multiple $C9$ proteins assemble at this point, forming a pore-like structure that creates a hole through the red cell membrane. This complex is known as the Membrane Attack Complex (MAC).

• Cell Lysis Mechanism:

  • Loss of Control: The MAC pore allows uncontrolled passage of various plasma constituents into the red cell, circumventing the normal regulation by active membrane transporter proteins.

  • Primary Damage: An excess of water molecules and sodium ions $(Na^+)$ entering the red cell causes the most damage.

  • Consequence: The cell swells uncontrollably and eventually bursts, releasing its contents. This is considered an