Comprehensive Study Guide to the Complement System

The complement system consists of a network of more than $20$ inert proteins that circulate throughout the blood and various tissue fluids.
Origin and Synthesis: These proteins are synthesized primarily by liver cells ( $hepatocytes$ ). Other sites of synthesis include $monocytes$ , $macrophages$ , and the $epithelial\,cells$ of the gastrointestinal tract.
Serum Concentration: These components constitute approximately $5\%$ of serum globulins.
Functional State: In their circulating state within the blood, they are functionally inactivated and exist as $proenzymes$ .
Mechanism of Action: The system operates as a biological cascade. This is defined as a series of highly interacting proteins where each component serves as a catalyst for the activation of the subsequent protein in the sequence.

Designation Methods: Components are identified using three naming conventions: - Numerals: Designated as $C1$ through $C9$ . - Letter Symbols: Including factors such as $Factor\,D$ , $Factor\,B$ , $Factor\,H$ , and $Factor\,I$ . - Trivial Names: Such as the $homologous\,restriction\,factor$ .
Cleavage and Fragments: When a complement component is cleaved, it results in smaller and larger fragments: - Smaller Fragment: Generally designated with the suffix "a" (e.g., $C3a$ , $C4a$ ). - Larger Fragment: Generally designated with the suffix "b" (e.g., $C3b$ , $C4b$ ). - Major Exception: In the cleavage of component $C2$ , the fragment designated $C2a$ is the larger fragment, while $C2b$ is the smaller fragment.
Functional Complexes: When fragments interact to form enzymatic complexes, they are designated with a bar over the numerical symbol (e.g., $\overline{C4b2a}$ ).

The activation of the complement system occurs via three distinct pathways: 1. Classical Pathway: Triggered by an antigen-antibody reaction. 2. Alternative Pathway: Triggered directly on the surfaces of microbial cells. 3. Lectin Pathway (or Mannose-binding Lectin Pathway): Triggered when plasma lectin binds to mannose residues located on microbes.

Trigger: The pathway is initiated by antibody-antigen complexes.
Initiation Step: $C1$ binds to the $Fc\,region$ of an antibody that has already interacted with an antigen.
Structure of $C1$ : It is a complex comprising one moiety of $C1q$ and two moieties each of $C1r$ and $C1s$ .
Sequential Cleavage: - $C1r$ and $C1s$ facilitate the cleavage of $C4$ and $C2$ . - $C4$ is divided into $C4a$ and $C4b$ . $C4a$ diffuses away from the site. - $C2$ is divided into $C2a$ and $C2b$ . $C2b$ diffuses away from the site.
$C3\,Convertase$ Formation: The fragments $C4b$ and $C2a$ combine to form the $\overline{C4b2a}$ complex, known as $C3\,convertase$ .
$C3$ Activation: $C3\,convertase$ divides $C3$ into $C3a$ and $C3b$ . $C3a$ diffuses away.
$C5\,Convertase$ Formation: Some $C3b$ molecules bind to the existing $C3\,convertase$ to form $\overline{C4b2a3b}$ , which is the $C5\,convertase$ .
$C5$ Activation: $C5\,convertase$ divides $C5$ into $C5a$ and $C5b$ . $C5a$ diffuses.
Membrane Attack Complex (MAC) Assembly: - $C5b$ binds to $C6$ to form the $C5bC6$ complex. - $C5bC6$ binds to $C7$ to form the $C5bC6C7$ complex. - $C5bC6C7$ binds to $C8$ to form the $C5bC6C7C8$ complex. - This final complex activates $C9$ , completing the formation of the $MAC$ .
Mechanism of Lysis: The $MAC$ inserts into the outer membrane of bacteria, killing them via lysis.

Trigger: Activated by microbial surface elements, such as $lipopolysaccharide$ . It is notable for not requiring an antigen-antibody complex for initiation.
Phase Importance: This pathway is critical during the early stages of an infection when microbial loads are high but antibody concentrations remain low, limiting classical pathway activation.
Execution Steps: - $C3$ travels to the infection site and activates upon contact with microbial surfaces. - $C3$ divides into $C3a$ and $C3b$ . - $Factor\,B$ and $Factor\,D$ interact with each other and with $C3b$ . - $Factor\,D$ cleaves $Factor\,B$ into $Ba$ and $Bb$ . - This forms the alternative $C3\,convertase$ , designated as $C3bBb$ . - Following this, the process mirrors the classical pathway to generate $C5\,convertase$ and eventually the $MAC$ .

Trigger: Circulating lectin binds to mannose residues on the surface of target cells or microorganisms.
Mannose-binding Lectin (MBL): Characterized as an acute-phase protein antibody. Its concentration increases significantly during inflammatory responses.
Protease Involvement: Upon binding to mannose, $MBL-associated\,serine\,proteases$ known as $MASP-1$ and $MASP-2$ are activated.
Similarity to Classical Pathway: $MASP$ enzymes activate $C4$ and $C2$ to form the $C4b2a$ ( $C3\,convertase$ ).
Outcome: The reaction sequence follows the standard procedure to generate $C5\,convertase$ and the $MAC$ .

Chemotaxis: Defined as the movement of cells in response to chemical stimuli. Complement receptors for $C5a$ and $C3a$ located on $neutrophils$ and $macrophages$ serve to direct these cells to the site of inflammation.
Opsonization: $C3b$ binds to the surface of pathogens or immune complexes. This fragment subsequently binds to phagocyte receptors, significantly enhancing उनकी phagocytic activity.
Cell-Lysis: The $MAC$ directly causes the rupture and lysis of microbial cell surfaces.
Activation of Mast Cells: The fragments $C3a$ , $C4a$ , and $C5a$ bind to mast cells. This induces degranulation, leading to the release of inflammatory mediators including $histamine$ , $heparin$ , $cytokines$ , and various $growth\,factors$ essential for immune response.
Antibody Production Enhancement: $B\,cells$ possess receptors specifically for $C3b$ . When $C3b$ binds to these receptors, it triggers the $B\,cells$ to secrete increased amounts of antibodies.
Immune Clearance: The system plays an anti-inflammatory role by solubilizing immune complexes and facilitating their removal from circulation. These complexes are then redirected for storage in the $spleen$ and $liver$ or cleared via $phagocytosis$ .

Systemic Lupus Erythematosus (SLE): Linked to deficiencies in $C2$ and $C4$ .
Pyogenic Bacterial Infections: Caused by a lack of $C3$ and $Factor\,D$ .
Neisseria Infections: Deficiencies in the late-acting components $C5$ , $C6$ , $C7$ , $C8$ , or $C9$ (collectively referred to as $MAC\,deficiency$ ) result in susceptibility to $Neisseria\,gonorrhoea$ and $Neisseria\,meningitis$ .
Regulatory Mutations: Mutations in complement regulator factors can cause $atypical\,hemolytic\,uremic\,syndrome$ and $hereditary\,angioedema$ .
Tissue Damage: The accumulation of $autoantibodies$ , $immune\,complexes$ , or $persistent\,microorganisms$ in tissues can trigger complement activation that leads to localized tissue damage.