Immunology part 2 (trans 2) Flashcards
COMPLEMENT SYSTEM
- Part of 2nd line of defense (inflammation, phagocytosis, complement)
- A set of proteins that play a role in cytolytic destruction of cellular antigens by specific antibody
- Most complement components are synthesized in the liver except C1 (intestinal epithelial cells); Factor D (adipose); C1, C2, C3, C4 are also synthesized by macrophages
Destruction of Complement Proteins
- Complement should be destroyed because it interferes with serological tests
1. Heat inactivation – heat the serum at 56C for 30 mins
2. Anticoagulant (ex: EDTA) – binds Ca2+ to make it unavailable; C1qrs (a trimolecular complex) is stabilized by Ca2+ so if C1 is destroyed, the complement is destroyed
3. Prolonged storage – complement is only present in fresh serum; aging serum destroys complement
clinical correlation
Hypocomplementemia – characterized by low levels of complement especially C3; clinically significant specially in diagnosing autoimmune diseases
C3: most abundant component of the complement system; used to detect hypocomplementemia
Complement activation pathways
- Classical Pathway – triggered after Abs bind to microbes or other Ags; component of the humoral arm of adaptive immunity because it involves Abs
- Alternate/Properdin Pathway – complement proteins are activated on microbial surfaces; component of the innate immunity
- Lectin Pathway/Mannan-binding Leptin Pathway – binding to terminal mannose on surface glycoproteins of microbes; component of the innate immunity
Classical pathway
- C1q is attached to the Fc fragment of antibody
- C1qrs cleaves C4 yielding C4a and C4b
- Activation of C2 yields C2a and C2b
- C4b + C2a => C4b2a (C3 convertase)
- C3 convertase converts C3 into C3a and C3b
- C4b2a + C3b => C4b2a3b (C5 convertase)
- C5 convertase converts C5 into C5a and C5b
- C5b + C6 + C7 + C8 = slow lysis
- C9 enters through pores in the surface of bacterial cell
- C5b + C6 + C7 + C8 + C9 (membrane attack complex /MAC) = rapid osmotic lysis
- *C5a - most potent anaphylatoxin; responsible for increased vascular permeability, pain induction, muscle contraction and inflammatory response
- *Recognition unit: C1qrs particularly C1q binds to Ab - stabilized by Ca2+
- *Activation unit: formation of C5 convertase - C4, C2, C3
- *MAC: C5-C9
Alternative pathway
- *note it bypasses C1, C4 and C2
1. Factor B is activated into Factor Ba and Bb (Factor Bb is the active form) with the participation of C3, factor D and Mg2+ ions
2. Factor Bb will cleave C3 into C3a and C3b and will combine with C3b to form C3bBb (C3 convertase of the alternate pathway)
3. C3bBb + C3b => C3bBb3b (C5 convertase)
4. Properdin (Factor P) – binds to C3bBb to prevent spontaneous decay of the complex
5. MAC
**recognition unit (does not require Abs):
- Bacterial cell walls (w/ LPS)
- Fungal cell walls
- yeasts
- viruses
- virally infected cells
- tumor cell lines
- some parasites
**activation unit:
C3 convertase - C3bBb
C5 convertase - C3bBb3B
**C5 convertase and even starting from C3 convertase is stabilized by properdin (increases half-life from 90 sec to minutes)
(Mannan-binding) lectin pathway
- Almost the same in classical pathway; difference is how they are triggered
- Activated by mannose groups of glycoprotein
- Effectors:
1. MBP/MBL – Mannan-Binding Protein/Lectin
2. MASP – MBP-Associated Serine Protease
- *recognition unit: MASP-1 complex attaches to mannose/mannan, glycoproteins on bacteria, yeasts viruses and some parasites
- *Activation unit: C5 convertase – C3b4b2a
- *Lectin has Ab-like activity that binds to CHO
- *Lectin pathway plays an important role in infants during loss of maternal Ab and acquisition of full-fledged Ab response to pathogens
MBL – binds to mannose or related sugars in Ca-dependent manner
Deficiency in MBL – common in infants w/ recurrent respiratory tract infections, otitis media and chronic diarrhea
what are the major biologic effects of complement?
1 . Opsonization - efficient phagocytosis
- Chemotaxis - C5a stimulates movement of neutrophils and monocytes toward sites of Ag deposition
- Anaphylatoxins - C3a and C5a can produce increased vascular permeability and smooth muscle contraction; C3a and C5a also stimulates mast cells to release histamine
- Cytolysis
Chemotaxins
C5a, C5b, C6, C7
Immune adherence
C3b
Kinin Activator
C2b
Anaphylatoxins
C3a, C4a, C5a (most potent)
Opsonins
C3b, C4b, C5b
Virus neutralization
C4b, C1
Plasma Complement Regulation
- C1 inhibitor (C1INH) – dissociates C1r and C1s from C1q
- Factor I – cleaves C3b and C4b
- Factor H – competes for Factor B; regulator of alternative pathway
- C4 binding protein – acts as a cofactor with I to inactivate C4b
- S protein (vitronectin) – prevents attachment of the C5b67 complex to cell membrane
- Decay accelerating factor (DAF) or CD55 – accelerates dissociation of C3 convertase
Inhibitors of MAC
- HRF – Homologous Restriction Factor
- CD59 or MIRL (Membrane Inhibitor of Reactive Lysis) – will act on MAC
clinical correlation
PNH or Paroxysmal Nocturnal Hemogobinuria
- Absence of CD55 and CD59
- Complement-mediated hemolysis
- Hemoglobinuria (due to destruction of RBCs) and hematuria may occur
- Nocturnal: named as such not because of night time but it is the shift of the pH of the body when we sleep
- The only acquired intrinsic normocytic hemolytic anemia; the rest are hereditary
- Usually patients develop myelodysplastic syndrome or preleukemia syndrome
Deficiencies of Complement Components
- C1 - LE-like syndrome
- C2 - LE-Like syndrome, recurrent infections
- C3 - severe recurrent infection
- C4 - lupus-like syndrome
- C5, C6, C7, C8 - Neisseria Syndrome
- C9 - no known disease
- C1INH - Hereditary Angioedema (HANE)
- DAF and HRF - Paroxysmal nocturnal hemoglobinuria
- Factor H or I - recurrent bacterial infections
Disorders of the immmune system
- Hypersensitivity
- Autoimmunity
- Immunodeficiency
- Amyloidosis
Disorders of the immmune system
Hypersensitivity
- a condition in w/c an immune response results in exaggerated or inappropriate reactions harmful to the host
- such reactions typically occur after the second contact with a specific antigen (allergen)
- the first contact is a necessary preliminary event that induces sensitization to that allergen
Disorders of the immmune system
Hypersensitivity: Type 1- IgE-Mediated Hypersensitivity/ Early Immediate Anaphylactic/ Immediate Hypersensitivity
Key characteristic: Fast response occurs within minutes
Initiation: 2-30 mins.
Mechanism: IgE found on mast cells cross link with antigen thus causing mast cells to degranulate releasing vasodilators (histamine)
Example: food and drug allergy, bee stings, systemic and local anaphylaxis, hay fever, asthma, eczema
Other notes:
o Wheal and flare reaction
o Can be systemic or localized (localized to a specific organ)
o Can cause anaphylactic shock which is lethal
o Food allergy may be as benign as rash and as serious as bee sting
Disorders of the immmune system
Hypersensitivity: Type 2- Cytotoxic Hypersensitivity/Antibody-mediated Cytotoxic Hypersensitivity
Key characteristic: Antibody directed against cell surface antigens mediates cell destruction via Antibody Dependent Cell mediated Cytotoxicity (ADCC) or complement
Initiation: 5-8 hrs.
Mechanism: IgG or IgM bound to cell surface antigens, with subsequent complement fixation. Produces direct damage to cell surface.
Test: direct and indirect Coombs test
Example: Blood transfusion reactions (Type A blood cell transfused into Type B blood causing Type A antibody to react to Type A blood cell), Hemolytic disease of the newborn (Erythroblastosis Fetalis), Autoimmune hemolytic anemia
Disorders of the immmune system
Hypersensitivity: Type 2- Cytotoxic Hypersensitivity/Antibody-mediated Cytotoxic Hypersensitivity
**Erythroblastosis fetalis where there is no problem with the first baby but the mother (Rh-) has developed immunity (Anti-Rh antibodies) on Rh+ baby. The second baby born has hemolysis which leads to elevated bilirubin levels. Unconjugated bilirubin has high affinity for fatty tissues so if bilirubin levels reach >20mg/dL, it binds to the neonatal brain (where most fat in babies are) thus causing kernicterus. Neonate undergoes phototherapy to conjugate bilirubin to water soluble form.
To prevent Erythroblastosis fetalis, mothers are injected with rhogam (antibodies that eliminate Rh+ RBC in maternal blood)
- May be caused by Rh (no problem with first child or ABO (problems can arise in first child)
- Do coombs test to check
Hypersensitivity: Type 2- Cytotoxic Hypersensitivity/Antibody-mediated Cytotoxic Hypersensitivity
Autoimmune Hemolytic Anemia
4 types:
Warm agglutinin (70%) *Agglutinin aka antibody
Cold agglutinin (antibodies that work best at 4oC)
Mixed agglutin
Paroxysmal cold hemoglobinuria : upon exposure to cold, there is hemoglobinuria
*In this type of hemolytic anemia, the body thinks that there is an intruder when in fact there’s none (autoimmune). They consider some of the normal cells as non-self.
Hypersensitivity: Type 3 - Immune Complex Mediated Hypersensitivity
Key characteristic: immune-complex reactions
Initiation: 2-8 hrs.
Mechanism: Antigen- antibody complexes deposited at various sites induces mast cell degranulation via Fc gamma RIII; PMN degranulation damages tissue
Example: Arthus Reaction (local reaction), systemic reactions disseminated rash, arthritis, glomerulonephritis, systemic lupus erythromatosus
Hypersensitivity: Type 4 – Delayed/Cell-mediated Hypersensitivity
Key characteristic: delayed hypersensitivity reactions, cell-mediated immunity, mediated by T-cells
Initiation: 24-72 hrs
Mechanism: Memory TH1 cells release cytokines that recruit and activate macrophages.
Example: Contact dermatitis, tubercular lesions, organ transplant rejection
Other notes:
o Opposite of type I
o First three types are humoral, only Type 4 is cell-mediated.
o Tuberculin test will be positive on people infected or vaccinated with M. tuberculosis, swelling on part of skin tested.
o Corneal transplant – organ least likely to have a transplant rejection due to absence of blood supply