Lectin Pathway and Biological Roles of the complement System

The complement system can be activated through three pathways: ●​ Classical pathway ●​ Alternative pathway ●​ Lectin pathway What is the Lectin Pathway? The lectin pathway is one of the three complement activation pathways and is an important part of the innate immune system. Unlike the classical pathway, it does not require antibodies and is activated when mannose-binding lectin (MBL) or ficolins recognize carbohydrate molecules on the surface of pathogens. Why is it called the Lectin Pathway? The lectin pathway is named after lectins, which are proteins that bind specific carbohydrate molecules. Mannose-binding lectin (MBL) recognizes mannose residues commonly found on microorganisms but rarely on human cells, allowing the immune system to distinguish pathogens from host tissues. Key points: ●​ Antibody-independent pathway ●​ Initiates early immune defense ●​ Recognizes microbial carbohydrates Notes: ●​ ●​ ●​ ●​ Lectins are soluble proteins They recognize specific carbohydrate components on microbial surfaces MBL- mannose- binding lectin was the first lectin to be discovered MBL binds to mannose containing carbohydrates of bacteria, fungi, protozoa, and viruses ●​ MBL-Associated Serine Protease Components of the Lectin Pathway Several proteins work together to activate the lectin pathway. Mannose-binding lectin (MBL) recognizes specific sugars on microbial surfaces, while MASP-1 and MASP-2 are enzymes that trigger the complement cascade by cleaving complement proteins C4 and C2. Component Function MBL Recognizes mannose residues on pathogens Ficolins Detect carbohydrate patterns on microbes MASP-1 Assists in activating MASP-2 MASP-2 Cleaves C4 and C2 to form C3 convertase Mechanism of the Lectin Pathway The lectin pathway begins when MBL binds to mannose residues on the pathogen surface. This activates MASP proteins, which cleave complement proteins C4 and C2 to form C3 convertase (C4b2a). C3 convertase then cleaves C3 into C3a and C3b, leading to the formation of C5 convertase and C5b binds sequentially to C6, C7, C8, and multiple C9 molecules to form the Membrane Attack Complex (MAC), which creates pores in the microbial membrane and causes cell lysis. Functions of the Lectin Pathway Activation of the lectin pathway contributes to host defense in several ways. It helps eliminate pathogens by coating them with C3b (opsonization), recruiting immune cells through inflammatory mediators (C3a and C5a), and directly destroying microbial cells through the membrane attack complex. Major functions: Opsonization Inflammation Cell lysis Clinical Significance The lectin pathway provides rapid protection against infections before antibodies are produced, making it especially important during the early stages of infection. Deficiency of mannose-binding lectin may increase susceptibility to recurrent bacterial, viral, or fungal infections, particularly in infants and immunocompromised individuals. Comparison of the Three Complement Pathways Although all three complement pathways activate the same terminal pathway, they differ in how they are initiated. The lectin pathway resembles the classical pathway in its mechanism but is activated by recognition of microbial carbohydrates rather than antibodies. Classical Lectin Alternative Triggered by antigen-antibody complexes Triggered by MBL binding to mannose Triggered by spontaneous C3 activation Requires antibodies Does not require antibodies Does not require antibodies Does not require antibodies Adaptive immunity Innate immunity Innate immunity Summary The lectin pathway is an antibody-independent mechanism of complement activation that plays a vital role in innate immunity. By recognizing carbohydrate molecules on pathogens, it initiates a cascade of complement reactions that promote inflammation, enhance phagocytosis, and destroy microorganisms through the membrane attack complex. The lectin pathway serves as a rapid first-line defense against pathogens before the adaptive immune system produces antibodies. VII. Biological Roles of the Complement System What does the complement system actually do after it's activated? Think of the complement system as a special defense team. Once it's activated, it doesn't just perform one function—it carries out four major biological roles that work together to eliminate pathogens and protect the body. You can think of these as its four missions: ●​ Mission 1: Call for Help (Inflammation) ●​ Mission 2: Tag the Enemy (Phagocytosis/Opsonization) ●​ Mission 3: Clean Up the Battlefield (Immune Complex Clearance) ●​ Mission 4: Finish the Job (Bacterial Osmotic Lysis) A. Complement in Inflammatory Responses Mission 1: Call for Help Purpose The complement system initiates inflammation and recruits immune cells to the site of infection. Mechanism: ① Pathogen enters the body Bacteria or other microorganisms invade the body, activating the innate immune response. ⬇ ② Complement system is activated The classical, lectin, or alternative pathway activates the complement cascade. ⬇ ③ C3a and C5a are released These proteins, called anaphylatoxins, function as chemical alarm signals that alert the immune system. ⬇ ④ Blood vessels dilate and become more permeable More plasma proteins and immune cells leave the bloodstream and enter the infected tissue. ⬇ ⑤ Neutrophils migrate to the infection C5a attracts neutrophils and other leukocytes through chemotaxis, guiding them to the pathogen. ⬇ ⑥ Inflammation occurs The infected area becomes red, warm, swollen, and painful as immune cells begin destroying pathogens. Importance ●​ Initiates the body's first line of defense. ●​ Delivers immune cells rapidly to infected tissues. ●​ Helps prevent the spread of infection during the early stages. ●​ Easy to Remember ○​ C3a & C5a = Fire Alarm ○​ They call immune cells to the site of infection. B. Complement in Phagocytosis Mission 2: Tag the Enemy Purpose The complement system enhances pathogen recognition, making phagocytosis faster and more efficient. Mechanism: ① Bacteria enter the body Some bacteria are difficult for immune cells to recognize because of protective outer structures. ⬇ ② C3b binds to the bacterial surface C3b coats the bacterium through opsonization, acting like a "tag." ⬇ ③ Phagocytes recognize C3b Macrophages and neutrophils use Complement Receptor 1 (CR1) to recognize the tagged bacterium. ⬇ ④ Phagocytosis occurs The phagocyte engulfs the bacterium into a phagosome. ⬇ ⑤ Intracellular destruction Lysosomal enzymes and reactive oxygen species digest and destroy the bacterium. Importance ●​ Makes bacteria easier to recognize. ●​ Increases the efficiency of bacterial clearance. ●​ Reduces the chance of bacterial survival and multiplication. ●​ Easy to Remember ○​ C3b = Target Sticker ○​ It marks bacteria so phagocytes can easily find and destroy them. C. Complement in the Elimination of Immune Complexes Mission 3: Clean Up the Battlefield Purpose The complement system removes immune complexes from circulation to prevent tissue damage. Mechanism: ① Antibodies bind to antigens This forms immune complexes, which are a normal part of the immune response. ⬇ ② Immune complexes circulate If not removed, they may accumulate in tissues and trigger inflammation. ⬇ ③ C3b binds to the immune complexes C3b coats the complexes, preparing them for removal. ⬇ ④ Red blood cells transport the complexes Erythrocytes use CR1 receptors to carry the immune complexes to the liver and spleen. ⬇ ⑤ Macrophages remove the complexes Macrophages in these organs engulf and degrade the immune complexes. Importance ●​ Prevents immune complex deposition in tissues. ●​ Reduces inflammation and tissue injury. ●​ Helps prevent autoimmune diseases such as Systemic Lupus Erythematosus (SLE). ●​ Easy to Remember ○​ Complement = Cleanup Crew ○​ It removes immune complexes before they can damage healthy tissues. D. Complement in Bacterial Osmotic Lysis Mission 4: Finish the Job Purpose The complement system directly destroys bacteria that escape phagocytosis. Mechanism: ① Complement activation reaches the terminal pathway The complement cascade proceeds to its final stage. ⬇ ② Membrane Attack Complex (MAC) assembles Complement proteins C5b, C6, C7, C8, and multiple C9 molecules assemble on the bacterial membrane. ⬇ ③ A pore forms in the bacterial membrane The MAC creates a channel that disrupts the bacterial membrane. ⬇ ④ Water enters the bacterium Water and ions rush into the cell because of osmotic pressure ⬇ ⑤ The bacterium swells The increasing internal pressure causes the bacterium to expand. ⬇ ⑥ Bacterial osmotic lysis The bacterial membrane ruptures, causing the bacterium to burst and die. Importance ●​ Provides direct killing of bacteria, especially Gram-negative bacteria. ●​ Acts as the complement system's final defense mechanism when pathogens escape phagocytosis. ●​ Easy to Remember ○​ MAC = Hole Puncher ○​ It punches holes in bacteria until they burst. Overall Importance of the Complement System The complement system is one of the body's most important innate defense mechanisms. It not only attacks pathogens directly but also works together with antibodies and immune cells, making it a crucial bridge between innate and adaptive immunity. Through its four biological roles, the complement system: ●​ Recruits immune cells to sites of infection. ●​ Enhances pathogen recognition through opsonization. ●​ Removes immune complexes to prevent tissue damage. ●​ Directly destroys bacteria through the Membrane Attack Complex. ●​ Without a functional complement system, individuals become more susceptible to recurrent infections and immune-related disorders. Conclusion The complement system is much more than a group of proteins circulating in the blood—it is a coordinated defense team that protects the body through four essential missions. Together, these coordinated actions allow the complement system to rapidly detect, contain, eliminate, and clear pathogens, making it an essential component of both innate immunity and the body's overall immune defense.