COMPLEMENT PART 1

• Complement system


  Humoral barrier of infection, 2nd line of defense, major humoral non-specific defense mechanism
• Complement system activation


  1. May lead to vascular permeability

  2. Lysis

  3. Opsonization

  4. Recruitment of phagocytes
• Complement


  System of plasma proteins which completes the process initiated by the interaction of antigen and antibody combination
• Complement


  • Consists of approximately 20 proteins present in normal human serum
  • Proteins are produced in the liver
  • C1 is in the intestinal epithelial cells
  • Factor D is produced in the adipose cell
  • Heat-labile
  • Important component of the innate (natural) host defense / immune system

  
• Complement participates in Type 2 and 3 hypersensitivity
• Refrigerator temperature


  Activity is lost in 3-4 days, do not put specimen at refrigerator temperature when performing complement protein tests
• Room temperature


  Complement deteriorates within 1-2 days
• Best reaction conditions


  • pH level: 7.2-7.4
  • Temperature: 30-37°C
  • In the presence of: Ca & Mg
• Functions of complement


  • Cell lysis by the MAC
  • Effector functions of components (C5a,C3a,C4a,C5a,C3b)
  • Increase vascular permeability
  • Recruit monocytes and neutrophils to the area of antigen concentration
  • Trigger secretion of immunoregulatory molecules that amplify the immune response

  
• Complement proteins

  • Often designated by an uppercase letter C and are inactive until they are split into products, to become active
  • The active products are usually designated with a lowercase a or b
• Immunoglobulins that fix complement

  • IgM
  • IgG3
  • IgG1
  • IgG2
• Immunoglobulins that do not fix complement

  • IgG4
  • IgA
  • IgD
  • IgE
• Complement activation pathways


  • Classical pathway
  • Alternative pathway
  • Lectin pathway

  
• Three main stages of complement activation

  • Recognition unit
  • Activation unit
  • Membrane attack complex (MAC)

  
• Three anaphylatoxins


  • C3a
  • C4a
  • C5a
• C5a


  Chemotaxin and anaphylatoxin
• Initiators of the three complement activation pathways

  • Classical pathway: Immune complexes, apoptotic cells, certain viruses and gram-(-) bacteria, C-reactive protein bound to ligand
  • Alternative pathway: Various bacteria, fungi, viruses, or tumor cells
  • Mannose-binding lectin pathway: Microbes with terminal mannose groups

  
• Classical pathway


  • Part of adaptive immunity, occurs at the latter portion of immunity since producing antibody takes time
  • Activated by antibodies bound to the antigen
  • Igs involved: IgM and IgG
  • Involves activation of C1: binds to the Fc portion of IgM and IgG and requires Ca for activation
  • Substances that can bind complement directly to initiate classical pathway: CRP, gram-(-) bacteria, mycoplasmas, protozoans, several viruses

  
• Flow of the classical pathway

  1. C1 binds to either IgM or IgG

  2. C1s cleaves C4 into C4a and C4b

  3. C1s cleaves C2 into C2a and C2b

  4. Formation of C5 convertase
• C3


  The major constituent of the complement system
• C4


  The 2nd most abundant complement protein
• Most significant biological consequences of the complement system: splitting of C5 and cleavage of C3
• C4 and C2 are magnesium-dependent
• Recognition unit of the classical pathway

  C1 binds to either IgM or IgG
• Activation unit of the classical pathway

  1. C1s cleaves C4 into C4a and C4b

  2. C1s cleaves C2 into C2a and C2b

  3. Formation of C3 convertase (C4b2a)
• C4b2a


  C3 convertase, active enzyme with a half-life of 3.15 minutes
• C2b


  Increases capillary permeability, leading to edema if activation is not controlled
• C3 convertase must be bound quickly since C3 needs to be cleaved to C3a and C3b, with C3b acting as an opsonin which binds to C4b2a to form C5 convertase