Module 9 Flashcards
Clinical Applications of Immunology
hypersensitivity reactions
a detrimental effect of the immune system
- a reaction is termed hypersensitive either because it is a heightened or inappropriate response to an antigen
Gell-Coombs classification of immune injury
this system categorizes hypersensitivity reactions into 4 types: Type I, II, III, and IV
Type I hypersensitivity
Type I immediate hypersensitivity reactions can develop into anaphylactic responses
- type I is a specific antibody mediated reaction
- IgE on the surface of mast cells binds an allergen, resulting in mast cell degranulation
- some of the complement reaction byproducts (C3a, C4a, and C5a) can also cause this type of reaction
type II hypersensitivity
mediated by antibody directed against cell surface antigens
- AKA cytotoxic or costimulatory hypersensitivity
Type II reactions result in complement mediated lysis or cytotoxic action by NK cells
type III hypersensitivity
mediated by Ab-Ag immune complex deposited on the tissue
these immune complexes can activate complement and recruit innate immune cells to cause tissue damage
type IV hypersensitivity reactions
involve a complex interaction of T cells and monocytes/macrophages
- called delayed hypersensitivity (DHT) reactions
immediate/anaphylactic hypersensitivity
a type I reaction is an immediate response to an allergen
- most of these allergens are enzymes or glycoproteins.
- typical allergens include Fel d1 and Fel d4, Fra a1, der p1
sensitization
may occur via many routes including oral ingestion, respiratory inhalation, skin absorption, and IV
- the reaction is a result of IgE-mediated mast cell degranulation and are T cell dependent
type I hypersensitivity mechanism
- APCs present the processed allergen to Th2 cells. activated Th2 cells secrete IL-4 and IL-13, which induce naive B cells to undergo class switching from IgM to IgG2 to IgE secreting B cells
- secreted IgE binds to the F c-epsilon receptor on mast cells. the allergen cross-links two IgE antibodies bound to mast cells, resulting in mast cell degranulation
- mast cell degranulation results in the release of mediators, including vasoactive amines, which elicit biological effects within minutes
type I: mast cell activation
activated mast cells released pre-formed molecules and synthesize new vasoactive amines, leading to both early and late phase biological effects
early phase biological effects
immediate biological effects on mast cell activation are the result of the release of pre-formed molecules, such as:
- histamine, which promotes vascular permeability and smooth muscle contraction
- eosinophil chemotactic factor, which attracts eosinophils
- neutrophil chemotactic factor, which attracts polymorphic nuclear cells such as mast cells, eosinophils, and neutrophils
- proteases, which increase mucus secretion and causes basement membrane damage, especially in lungs
late phase biological effects
these are the result of synthesis of new vasoactive amines such as:
- platelet activating factor, which promotes aggregation of platelets, further mast cell degranulation, and smooth muscle contraction
- leukotrienes, which promote smooth muscle contraction and increase vascular permeability
- prostaglandins which promote smooth muscle contraction and vasodilation
- Bradykinin: promotes smooth muscle contraction and vascular permeability; involved in many pain pathways
effects of type I hypersensitivity
mediators released by mast cells effect:
- Blood Vessels: vasodilation and increased permeability result in edema. this loss of fluid in tissues can lead to anaphylactic shock
- Skin: vasodilation and edema can result in urticaria or eczema
- Nose and Eyes: vasodilation and increased permeability result in edema, which can manifest as rhinitis and conjunctivitis
clinical examples of Type I hypersensitivity
atopy: a predisposition to developing an allergic reaction
asthma: a condition where airways narrow and swell
laryngeal edema: a condition characterized by swelling of the larynx
atopy
an exaggerated IgE-mediated immune response to an environmental allergen - an inherited condition with genetic association to HLA, and can lead to asthma, atopic dermatitis, and allergic rhinitis
asthma
a respiratory condition characterized by increase in mucus secretion, edema, and contractions of brochiolar smooth muscle
- inflammation and obstructed airways are characteristic of an asthmatic reaction
key pathological components of asthma
increased recruitment of inflammatory cells
increased recruitment of mast cells, leading to high levels of histamine
- asthma can be atopic (reaction occurs in part of body not in contact with allergen), which is mediated by systemic IgE production, or non-atopic (reaction occurs in part of body in contact with allergen), which is mediated by localized IgE production, and does not occur in response to an allergen
laryngeal edema
a life-threatening swelling of the larynx characterized by inspiratory stridor (high pitched breathing sound)
allergy diagnosis
a common diagnostic test for immediate hypersensitivity reactions are skin prick tests, which measure the presence of IgE antibodies against suspected and common allergens. a positive reaction occurs within 1-20 minutes and will manifest as a small, red swelling
- this is a diagnostic test for a typical allergen
type II: cytotoxic/co-stimulatory hypersensitivity
these reactions involve antibody-mediated destruction of cells presenting allergens on their surface
- in this reaction, antigen-specific antibodies bind to the cell surface antigen and destroy the cell
- type II reactions are implicated in autoimmune diseases where the cells targeted are the host’s own RBCs
cell death
can occur by one of three mechanisms:
1. complement mediated cell lysis
2. phagocytosis
3. antibody-dependent cell mediated cytotoxicity (ADCC)
type II reaction mechanism: ADCC
antigen-specific antibody binds to the antigen via the Fab region
- specifically, cytotoxic cells have the CD16 gene, which binds to the Fc region of the antibody
- this cross linking between the target cell and the cytotoxic cell promotes cell killing due to the release of hydrolytic and digestive enzymes from the cytotoxic cells to the surface of the target cell
- the antibody does not directly kill target cells, but mediates cell death by presenting antigen cytotoxic cells
type II reaction: phagocytosis
antigen specific antibodies bind to the cell surface antigen via the Fab region. phagocytic cells have Fc receptors that bind to the Fc region of the antibody, cross-linking the antigen with phagocyte cells to enhance the process of phagocytosis
- if complement is activated, it promotes C3b on the surface of the target cell that can bind to the C3b receptor on phagocytes for enhanced phagocytosis
type II reaction: complement mediated lysis of target cell
antibody binding to antigen presented on the cell surface forms an Ab-Ag complex that activates the complement system, ultimately ending in cell lysis through a MAC attack
clinical examples of Type II reactions
hemolytic anemia: a condition where the immune system recognizes its own RBCs as foreign
Bullous pemphigus: autoimmune disease resulting in formation of blisters in the space between dermis and epidermis
Transfusion reactions: adverse reactions to allogeneic RBCs following blood transfusion reactions
Rh disease: a hemolytic condition in which the mother and fetal blood are incompatible
autoimmune hemolytic anemia
a group of blood disorders characterized by fatigue, dyspnea, and pallor. this is a result of lysis of RBCs due to antibodies
- lysis of RBCs consequently decreases the number of oxygen carrying blood cells in circulation
immune cells attack both self-RBCs and allogeneic RBCs that come from outside the individual, such as those from a blood transfusion
hemolytic disease
newborn disease which develops when maternal IgG antibody specific to the Rh-D allele crosses the placenta and destroys fetal RBCs by binding to Rh-D and activating complement
- can be fatal to fetus, but treated with intrauterine blood exchange transfusion
hemolytic disease is prevented by the use of anti-Rh antibodies
type III: immune complex hypersensitivity
these reactions are mediated by immune complexes deposited in the tissue
- these complexes can damage various tissues, primarily through complement activation and recruitment of innate immune cells
type III inflammatory diseases
vasculitis
carditis
synovitis
dermatitis
glomerulonephritis
size of type III immune complex
depends on the ratio of antigen:antibody present
- this has implications on the immunogenicity of the complex
type III mechanism
- antigen:antibody complexes deposit in tissue or blood vessel wall
- immune complexes activate complement components and attract inflammatory cells
- vasoactive amines (histamine) released by basophils will increase the vascularity of the tissue
- prolonged complement activation results in tissue damage due to enzymes released by neutrophils
type III diagnostic application
the Arthus reaction is the skin test that detects an excess of local antibodies
- the reaction involves in situ formation of immune complexes after intradermal injection with an antigen
- if a patient has previously been exposed to the antigen and has circulation antibody, an Arthus reaction will occur
arthus reaction
not an immediate reaction and generally develops over 6-12 hours if antibody levels are elevated in circulation
clinical presentation of type III hypersensitivity
- serum sickness: reaction to proteins in antiserum derived from non-human animals
- Farmer’s lung: hypersensitivity pneumonitis - rheumatoid arthritis: a long term autoimmune disorder involving both Type II and III hypersensitivity
serum sickness
a reaction to proteins from an animal source which arises from the introduction of certain non-protein substances or following vaccine administration
- the immune system reacts to the foreign antiserum proteins by producing specific antibodies and forming immune complexes, which enter the blood vessels and activate complement cascade
- complement reaction results in hypocomplementemia and fatigue