ANSWERS TO IMMUNOPATHOLOGY LAB QUESTIONS

Question 1: What type of hypersensitivity reaction or immunologic mechanism is involved in this type of asthma? How is it mediated?

Answer: This is an example of a type I hypersensitivity reaction mediated by IgE antibodies (in humans). An allergen stimulates B lymphocyte production of IgE. IgE antibodies attach to mast cells and basophils. When the mast cell is re-exposed to the specific allergen, cross-linking of the IgE molecules leads to 1)mast cell degranulation with discharge of preformed (primary) mediators and de novo synthesis and release of secondary mediators.

Question 2: What cell is primarily involved in this reaction? Can you find them in the slide easily?

Answer: Mast cells, and to a lesser extent basophils, are the cells primarily involved. Mast cells are not easily identified in a routinely stained H&E slide since their characteristic granules do not stain; they are more easily seen in Toluidine blue or Giemsa stained sections.

Question 3: Why are eosinophils present?

Answer: Leukotriene B4 and primary mediators such as eosinophil chemotactic factors of anaphylaxis (ECF-A) attract eosinophils to the involved area.

Question 4: What is anaphylaxis? Name some examples of systemic anaphylaxis, local anaphylaxis. Why do the reactions occur where they do?

Answer: Anaphylaxis is a rapidly developing immunologic reaction occurring within minutes after the combination of an antigen with antibody bound to mast cells or basophils in individuals previously sensitized to the antigen.

Systemic anaphylaxis: injection of an antigen to which the host is already sensitized, e.g., heterologous proteins (antisera), hormones, enzymes, drugs.

Local anaphylaxis: angioedema, urticaria, allergic rhinitis, allergic gastroenteritis, allergic bronchial asthma.

Local reactions take place at the portal of entry of the allergen, principally at mucosal sites.

Question 5: How do type II, type III and type IV hypersensitivity reactions or immune mechanisms differ from type I?

Answer: Type I: (Anaphylactic) Formation of IgE (cytotropic) antibody release of vasoactive amines and other mediators from mast cells.

Type II: (Cytotoxic) Formation of IgG, IgM binds to antigen on target cell surface phagocytosis or lysis of target cell by activated complement or antibody-dependent cellular cytotoxicity (ADCC).

Type III: (Immune Complex) Antigen-antibody complexes - attracted neutrophils - release of lysosomal enzymes and other toxic substances.

Type IV (Cell-mediated or delayed hypersensitivity): Sensitized thymus-derived T lymphocytes - release of lymphokines and T cell mediated cytotoxicity.

Question 1: What is the evidence (clinically and histologically) for acute rejection?

Answer: Clinical: fever, sudden marked decrease in renal function.

Histologic: acute cellular rejection interstitial infiltrate of lymphocytes, plasma cells, macrophages, and eosinophils.

Acute vascular rejection - intimal thickening with endothelial cell swelling and vacuolization, fibrin deposition, inflammatory cell within the intima (endovasculitis or intimal arteritis), platelet-fibrin thrombi within glomerular capillaries, necrosis of vessel walls.

Question 2: What is the evidence (clinically and histologically) for chronic rejection?

Answer: Clinical: Mildly decreased renal function prior to this acute episode

Histologic: Thickening and fibrosis of arteries, interstitial fibrosis, globally sclerotic glomeruli and tubular loss.

Question 3: What types of immune mechanisms are involved in transplantation rejection? What are the main histologic features of each of these mechanisms as demonstrated in this slide?

Answer: Both cell-mediated (Type IV hypersensitivity) and circulating antibodies play a role. Antibodies may cause injury by several different mechanisms including antibody-dependent cell-mediated cytolysis, complement-dependent cytotoxicity (Type II) and deposition of antigen-antibody complexes (Type III).

Histologic manifestations

Cell-mediated (Type IV): interstitial infiltrate (predominantly lymphocytes), edema.

Antibody-mediated: vasculitis (intimal arteritis), thickened intima infiltrated by scattered neutrophils and mononuclear cells, mild interstitial hemorrhage and fibrin thrombi.

Question 1: Rheumatoid arthritis is thought to be what kind of immune disease?

Answer: Rheumatoid arthritis is thought to be an autoimmune disease with the major target organ being the joint synovium leading to a proliferative synovitis which invades and erodes cartilage and bone.

Question 2. What is the rheumatoid factor?

Answer: IgM antibodies to the Fc portion of IgG molecules (found in 80% of RA patients).

Question 3. What other immune component may be activated in rheumatoid arthritis?

Answer: A cellular mechanism with activation of infiltrating T lymphocytes by some unknown antigen may be involved.

Question 4. What is the basic difference between diseases of autoimmune origin and hypersensitivity diseases, since, in each, injury is caused by immunologic mechanisms?

Answer: Autoimmune disorders result from immune reactions against "self" antigens whereas the hypersensitivity diseases are detrimental immune reactions directed at exogenous antigens.

Question 1: What is the basic immunologic abnormality in SLE?

Answer: Immunologic mechanisms in SLE are immune complex deposition and autoantibody tissue destruction.

Question 2: What are some of the characteristic antibodies that may be found in this patient's serum?

Answer: Antibodies commonly found in SLE include: ANA, anti-dsDNA, anti-phospholipid, SS-A(RO) Antinuclear antibodies can be grouped into: 1) antibodies to DNA, 2) antibodies to histones, 3) antibodies to nonhistone proteins bound to RNA, 4) antibodies to nucleolar antigens.

Question 3: Explain the patient's clinical findings.

Answer: The patient had renal disease related to her SLE. She developed renal failure leading to congestive heart failure with pulmonary edema (severe dyspnea) and generalized edema or anasarca.

Question 4: Is the patient's clinical history alone sufficient for a diagnosis of SLE? What is required for the diagnosis?

Answer: Several (11) criteria are proposed for the diagnosis of SLE. A patient must demonstrate 4 of 11 criteria for confirmation of the diagnosis. Criteria include: malar rash, discoid rash, photosensitivity, oral ulcers, arthritis, serositis, renal dysfunction, neurologic disorder, hematologic disorder, immunologic disorder, antinuclear antibody.

Question 5: What are some other organs that may be involved in SLE and what kinds of clinical or physical findings would be present?

Answer: Brain- cerebritis (infarcts, seizures, psychosis)

Skin - malar or discoid rash, photosensitivity

Blood cells- cytopenias

Heart- Libman-Sacks endocarditis

Joints- arthritis

Question 6: SLE is a classic example of multisystem disease of what type of origin? What are some other diseases in this type of disease group?

Answer: Autoimmune

Rheumatoid arthritis, Sjogren's syndrome, polymyositis, scleroderma.

Question 7: Are the histologic features in this slide diagnostic of SLE without any history or laboratory data?

Answer: No, the histologic patterns seen in the kidney are not specific for SLE.

Question 1: Which type of hypersensitivity reactions do each of the above immune mechanisms represent?

Answer: Antibody-dependent cell-mediated cytotoxicity type II

Direct T-cell killing or cytotoxicity - type IV

Antibody-dependent complement-mediated cytotoxicity type II

Immune complex deposition with a localized Arthus type reaction - type III

Question 2: What are major mechanisms which may be involved in the development of autoimmunity?

Answer: Autoimmunity results from loss of self-tolerance. Four possible mechanisms have been postulated.

Bypass of T-helper cell tolerance: a potential hapten (autoantigen) is complexed to a new carrier such as a drug and the complex is recognized by nontolerant T cells; an infectious agent may cross react with human tissue triggering an immune response to the organism and the antibody so formed may then damage tissue.

Idiotype Bypass Mechanisms: anti-idiotypic antibodies react with a hormone receptor and can mimic hormone action (Grave's disease)

Imbalance of T-suppressor-helper function; loss of suppressor T-cell function will contribute to autoimmunity and excessive T-cell help may drive B cells to increase antibody production.

Emergence of a sequestered antigen: a self-antigen that is completely sequestered is likely to be viewed as foreign if introduced into the circulation resulting in an immune response.

Question 3: What are some mechanisms of tissue damage in autoimmune disease and examples of disease showing those mechanisms?

Answer: Autoimmune tissue injury could be mediated by antibodies or by T-cell-mediated reactions. Both types II and III hypersensitivity mechanisms have been implicated.

Examples: Opsonization and phagocytosis of red blood cells in autoimmune hemolytic anemia. (type II)

Fixation of anti-basement membrane and complement to the GBM in Goodpasture's syndrome. (type II) Antibodies directed against 3-cells of pancreatic islets in insulin-dependent diabetes. (type II) Deposition of circulating immune complexes in SLE. (type III)

Formation of antireceptor antibodies-anti-acetylcholine receptors in myasthenia gravis; antibodies to TSH receptors in Grave's disease. (type II)

Answer: Fibrinoid necrosis of the vessel walls, accumulation of neutrophils centered around vessels, and perhaps thrombi in lumen. Rupture of the vessels may produce local hemorrhages.

Question 2: This slide is an example of what type of hypersensitivity reaction (immunologic tissue injury)? How does it produce the histologic features that are seen in the slide?

Answer: Type III; immune complexes precipitate in the vessel walls triggering an inflammatory reaction with activation of complement, release of inflammatory mediators, activation of neutrophils, release of lysosomal enzymes.

Question 3: What are other diseases in which the same type of hypersensitivity reaction is operative?

Answer: Some forms of glomerulonephritis

Acute serum sickness

SLE, RA

Question 4: How is the Arthus reaction like the double-diffusion-in-agar reaction?

Answer: When antigen and antibody are allowed to diffuse toward each other in agar a precipitation band forms when the antigen and antibody meet. similarly if antigen is injected into the skin it will diffuse toward vessels. The precipitation occurs in the walls of small vessels.