What are the 4 types of Immunopathology?
Think of the pneumonic ACID
- Anaphylactic & Atopic - Type I
- Cytotoxic - Type II
- Immune complex - Type III
- Delayed (cell mediated) - Type IV
What is the general method of Type II immunopathology?
- Type II causes cell destruction (cytotoxicity) by the action of immunoglobulin with complement or cytotoxic cells.
- IgM, IgG bind to fixed antigen on “enemy” (what the body thinks is enemy anyways) -> cell destruction
What are the 3 main mechanisms for Type II tissue damage?
Which of the three is the most common mechanism?
- Complement-Mediated Damage (Major)
- Stimulatory hypersensitivity (uncommon)
- Neutralization
Describe Complement-Mediated Damage
- Most common mechanism
- Pearl: Normal cells that are bound by antibody should alert you to complement
- Activation of complement leads to
- Lysis (C6-9 i.e. autoimmune hemolytic anemia)
- Opsonization for phagocytosis
- Phagocytic lysosomal enzymes and ROS
- Release of histamine specific for C5a, 4a, 3a
- Attraction of neutrophils which can damage tissue
Describe the mechanism for Stimulatory Hypersensitivity
- Mimicry of hormone that normally activates the receptor –> agonism of the receptor by the autoantibody
- Antibodies look like the ligand for a particular receptor. CONSTANT activation with none of the normal regulation methods
- E.g. - IgG autoantibody to TSH receptor leads to long-acting thyroid stimulation -> hyperthyroidism
- Normal feedback doesn’t work on releasing IgG (Graves disease)
Neutralization rxns
- Define
- Mechanism (general)
- Definition: Binding of antibody to an epitope resulting in inactivation, neutralization or abnormal activation
- Mechanism: Ab binding to a protein (e.g. toxin) can inhibit binding to substrate or alter conformation, resulting in loss of activity
- e.g. - Autoimmune Abs against hormone or neurotransmitter receptors can either block or activate the receptor
What is an example of a type II disease that effects muscle?
Give a general outline of disease
Myasthenia Gravis - muscle (Only disease that starts with an M)
- Characterised by progressive muscle weakness
- Body has an Ab against AChR (Acetylcholine Receptor)
- Specifically the alpha subunit - likely low yeild?
- Complement and neutrophil mediated
What is an example of a type II disease that effects the Lung & Kidney?
Goodpasture Syndrome: AutoAb to lung and kidney basement mebrane (BM) - form of vasculitis
- Type IV collagen antigen is shared between these to organs (we know b/c no other organs involved)
- Characterized as linear/sharp (not lumpy & bump) floresence slide
Recall: BM is collagenous non-living connective tissue framework upon which the endothelial cells of capillaries sit
Name two Type II diseases that affect the heart and give brief mechanism
Dressler Syndrome: After M.I. people make some autoAb against their heart - improves as heart heals
- Manifests as persistent cardiac pain, fever, malaise
Rheumatic Heart Disease (RHD): heart disease occurring shortly after a streptococcal infection
- Cross-rxn between a Group A Streptococcus M-protein antigen and heart’s laminin endothelial lining
Rheumatic fever is the same disease with more widespread manifestations, including in the skin and CNS
What type II disease effects RBC’s?
Autoimmune Hemolytic Anemia
- May follow viral infxtn, other autoimmune syndromes or cancer
- Many drugs can induce AIHA
- For paroxysmal cold hemoglobinuria the autoAbs bind to red cells only at 15⁰C
What type II disease effects Platelets?
Autoimmune Thrombocytopenic Purpura (ATP)
- General: Bleeding abnormalities due to platelet destruction by autoAbs
- Platelets opsonized for destruction in the spleen
- Treatment: splenectomy and immune suppression
What type II disease effects normal Thyroid fxn?
Grave’s Disease
- IgG antibody to TSH receptor on thyroid cells.
- Not enough TSH receptors close enough to activate C1q, so no complement activation.
- Mimics TSH and causes cells to secrete thyroid hormones (T3, T4)
- Normal feedback doesn’t work because it isn’t TSH doing the stimulating
What type II disease effects Pancreatic islets?
Juvenile diabetes (Type I Diabetes)
- AutoAbs to islet-associated antigens
- Technically diabetes is considered Type IV immunopathology
When would you use a direct antibody test for Good Pasture’s Syndrome
- Direct antibody test: if you have the patient’s tissues (and hopefully a control normal tissue!)
- Use antisera to detect presence of attached IgG on patient’s kidney
- Control normal kidney should NOT have attached IgG
- Where IgG is present, it should be linear (sharply defined) and attached to all of the exposed surface (BM)
When would you u se an indirect antibody test to diagnose Goodpasture’s Syndrome?
- Indirect antibody test: if you ONLY have the patient’s serum
- Expose the normal kidney (no Ab in it), to the patient’s serum
- Wash off unbound antibody
- Use antisera to detect the IgG now bound to the normal kidney
- Which immunopathology is characterised by “lumpy-bumpy” as it relates to an immunoflorescent pattern
Lumpy-bumpy: represents clusters of antigen-Ab complexes stuck in the BM b/c they are too big to be filtered. They are non-uniform and are representative of Type III immunopathology
What immunopathology is characterized by linear immunoflorescence patterns?
- Linear: Ab that directly attack BM (basement membrane) and so they can uniformly distribute along the membrane. These indicate Type II immunopathology
What Ab would you use in immunofloresence patterns to determine if either a Type II or Type III immunopathology was involved?
Anti-IgG:To see the IgG that is bound to the BM
What is the innocent bystander phenomenon?
Innocent Bystander Phenomenon
- Common mechanism
- Damage occurs because the normal tissue happens to be in proximity to or infected by the REAL slim-shady (foreign antigen)
- Ex: A drug adheres to red cells and you make Ab against the drug. Complement lyses the red cell, not the drug.
How can antibody-mediated tissue damage result from Cross-rxn of a foreign antigen with self?
Cross-reaction of a foreign antigen with self: (rheumatic fever, ATP)
- Damage results from Abs binding to self-cells
- Researchers can’t identify the antigens that get things started because once you have clinical symptoms, the initiator is gone.
How can antibody-mediated tissue damage result from Hybrid (foreign+self) antigen formation? aka - “Illicit Help Model”
PDF SPECIFICALLY STATES TO MAKE SURE YOU KNOW THIS MECHANISM. YOU HAVE BEEN WARNED.
- We have normally occuring anti-self B cells that have yet to be deleted
- Issue comes if a foreign antigen couples to self antigen
- Anti-self B cell could binds self antigen w/ coupled foreign antigen
- Foreign epitopes is presented to Tfh on Class II MHC
- TfH -> cytokines engages coreceptors
- B-cell is ctivated and secretes Ab against itself
How can antibody-mediated tissue damage result from exposure of a sequestered antigen?
Exposure of a sequestered antigen:
- There are self-antigens that the immune system doesn’t generally see (they are sequestered)
- If these get into the bloodstream, they can trigger an immune response because the system hasn’t negatively selected against those self-antigens
- E.g.: some men who get mumps end up sterile. The mumps break down the blood/testis barrier, and allowes immunization to normally sequestered sperm antigens.
How can antibody-mediated tissue damage result from Inadequency of regulatory T cells?
Inadequacy of regulatory T cells:
- Need a proper balance between 5 types of helper/regulatory T cells (do you remember them all?) for appropriate immune response
- Th1, Th2, Th17, Tfh, Treg
- May have this balance perturbed and cause the breakdown of self/non-self discrimination
How can antibody-mediated tissue damage occur from the emergence of a forbidden clone?
Emergence of a forbidden clone:
- “forbidden clone” - a clone that escapes normal clonal deletion. It matures and has reactivity with self.
- Seen in myasthenia gravis.
