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What is one of the functions of lectins in immunity?
Complement proteins only activate in the presence of an infection.
True
Name one of the pathways involved in complement activation.
C3 and C5 convertases
Lectins help inhibit the ________ and biofilm formation in microorganisms.
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Match the following immune functions with their corresponding actions:
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What role does lectin play concerning drug synergy?
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Complement proteins are synthesized primarily in the spleen.
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What are the main components of the complement system?
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Which of the following is a characteristic of Gram-negative bacteria?
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Exotoxins are primarily produced by Gram-negative bacteria.
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What type of pathogen must invade host cells to replicate?
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_________ are peptides that can provide antimicrobial defense by disrupting the membranes of pathogens.
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Which cytokine is NOT involved in the activation of NK cells?
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Match the following types of proteins with their functions in immunity:
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Facultative intracellular pathogens can only replicate inside the host cells.
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The process through which pathogens are recognized and destroyed by immune cells is called _________.
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What is the main difference between the classical and alternative pathways of the complement system?
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The lectin pathway of complement activation requires the presence of antibodies to function.
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List two types of patterns recognized by innate immune cells.
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Opsonization of microbial cells by complement components enhances ______.
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C3aR and C5aR receptors are primarily involved in which aspect of the complement system?
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Match the following complement pathways with their characteristics:
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What proteins do MBL bind to during the lectin pathway of complement activation?
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Regulatory proteins enhance the effects of the complement system.
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Study Notes
Lectins and Antimicrobial Activity
- Lectins bind to carbohydrates on the surface of microorganisms, leading to agglutination (clumping) of cells
- Agglutination reduces the motility of microorganisms and makes it easier for antimicrobial drugs to work
- Lectins interfere with the sites on a microorganism's surface responsible for attachment, inhibiting adhesion and biofilm formation
Complement System
- A collection of soluble proteins found in blood and other body fluids
- Works with the innate and adaptive immune systems to eliminate pathogens
- Discovered by Jules Bordet in the 1890s as a heat-labile substance in normal plasma
- Consists of over 30 plasma proteins primarily synthesized by the liver
- Circulates in an inactive form unless infection occurs
- Complement proteins interact to form different activation pathways
Complement System Functions
- Lysis of microbes, allografts, and tumors
- Opsonization (enhancing phagocytosis)
- Generation of inflammatory mediators to attract neutrophils
- Clearance of immune complexes and apoptotic cells
Overview of Complement Pathways
- Begins with proteins binding to pathogens, directly or via antibodies
- Conformational change leads to activation of enzymatic mediators
- Activation results in the generation of C3 and C5 convertases, which cleave C3 and C5
- C3 and C5 cleavage releases active components that mediate complement functions
Complement System Effector Proteins
- Can label an antibody-antigen complex for phagocytosis (opsonins)
- Initiate inflammation (anaphylatoxins)
- Bind to a pathogen and initiate the formation of the Membrane Attack Complex (MAC)
- Act through complement receptors on phagocytic cells, granulocytes, or erythrocytes
- Regulatory proteins limit complement effects by promoting degradation or preventing binding to host cells
Complement Activation Pathways
- Classical Pathway: Initiated by the formation of an antigen-antibody complex
- Alternative Pathway: Antibody-independent, part of the innate immune system, initiated by foreign cell surfaces
- Lectin Pathway: Antibody-independent, initiated by host proteins bound to microbes
Classical Pathway of Complement Activation
- Initiated by the binding of C1q to the antigen-antibody complex
- Activates C1r and C1s, which cleave C4 and C2
- C4b2a forms the classical pathway C3 convertase
- C3 convertase cleaves C3, generating C3b and C3a
- C3b binds to the complex and C5 convertase is formed (C4b2a3b)
- C5 convertase cleaves C5, releasing C5a and C5b
- C5b initiates the formation of MAC, leading to cell lysis
Alternative Pathway of Complement Activation
- Initiated by the spontaneous hydrolysis of C3
- C3b binds to a pathogen surface
- Factor B binds to C3b and is cleaved by factor D
- C3bBb forms the alternative pathway C3 convertase
- C3 convertase cleaves C3, resulting in more C3b molecules
- Positive feedback loop amplifies the pathway
- C3bBb3b forms the alternative pathway C5 convertase
- C5 convertase cleaves C5, initiating MAC formation
Lectin Pathway of Complement Activation
- Initiated by mannose-binding lectin (MBL) binding to carbohydrates on microbial surfaces
- MBL binds to the MASP family serine proteases
- MASP cleaves C2 and C4, forming the lectin pathway C3 convertase
- C3 convertase cleaves C3, leading to the amplification of the pathway and production of C5 convertase
- C5 convertase cleaves C5, initiating MAC formation
Opsonization and Inflammatory Response
- C3a and C5a (anaphylatoxins) bind to specific receptors on macrophages, neutrophils, basophils, eosinophils, and mast cells
- Binding leads to the release of proinflammatory mediators
- Opsonization occurs when C3b and IgG bind to a pathogen's surface
- Phagocytes use receptors (CR1, CR3, CR4) to bind to complement-opsonized pathogens
- Fc receptors on phagocytes bind to antigens opsonized by antibodies
Pattern Recognition by Innate Immune Cells
- Pathogen-associated molecular patterns (PAMPs): Repeating patterns expressed by most microorganisms
- Damage-associated molecular patterns (DAMPs): Endogenous molecules released from damaged/dying cells
- Pattern recognition receptors (PRRs) on innate immune cells recognize PAMPs and DAMPs
- Recognition activates the immune response, leading to pathogen elimination
Functions of Innate Immunity
- Key role in fighting infectious diseases
- First line of defense against pathogens
Infectious Diseases
- Caused by five main groups of agents: viruses, bacteria, fungi, protozoa, and helminths (worms)
- Pathogens can reside in different compartments of the body and require distinct defense mechanisms
- Intracellular pathogens often cause disease by damaging or killing cells they infect
- Obligate intracellular pathogens, such as viruses, must invade host cells to replicate
- Facultative intracellular pathogens, such as mycobacteria, can replicate both inside and outside cells
- Endotoxins are membrane compounds of Gram-negative bacteria
- Exotoxins are peptides secreted by Gram-positive bacteria
Epithelia: Physico-Chemical Barriers
- Epithelia form the skin and linings of the body's tubular structures (respiratory, urogenital, and gastrointestinal tracts)
- Contribute to innate immunity as a physical barrier against pathogens
- Perform a variety of functions including protection, absorption, secretion, and excretion.
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Description
Explore the roles of lectins and the complement system in the immune response. This quiz covers how lectins agglutinate microorganisms and how the complement system aids in pathogen elimination. Test your knowledge on these essential components of immunity.
