Allergy and the skin Flashcards
Define hypersensitivity
Immune response that causes collateral damage to self, exaggeration of normal immune sensitivity
What are the types of hypersensitivity reactions and there mediators?
Type 1: Ig-E
Type 2: IgG Mediated
Type 3: Immune complex mediated
Type 4: cell-mediated hypersensitivity
Describe the process of type I hypersensitivity
Ag induces crosslinking of IgE bound to mast cells and basophils with release of vasoactive mediators
Describe the process of type II hypersensitivity
Ab directed against cell surface antigens mediates cell destruction via complement activation or ADCC
Describe the process of type III hypersensitivity
Ag-Ab complexes deposited in various tissues induce complement activation and an ensuing inflammatory response mediated by massive infiltration of neutrophils
Describe the process of type IV hypersensitivity
synthesised Th1 cells release cytokines that activate macrophages or Tc cells which mediate direct cellular damage
What are the typical manifestations of type I hypersensitivity?
Typical manifestations include systemic anaphylaxis and localised anaphylaxis such as hay fever, asthma, hives, food allergies and eczema
What are the typical manifestations of type II hypersensitivity?
Typical manifestations include blood transfusion reactions, erythroblastosis fetalis, and autoimmune haemolytic anaemia
What are the typical manifestations of type III hypersensitivity?
Topical manifestations include localised arthus reaction and generalised reactions such as serum sickness, necrotising vasculitis, glomerulonephritis, rheumatoid arthritis, and systemic lupus erythematosus
What are the typical manifestations of type IV hypersensitivity?
Typical manifestations include contact dermatitis, tubercular lesions and graft rejection
Define allergy
Hypersensitivity disorder of the immune system
allergic reactions occur when a persons immune system reacts to normally harmless substances in the environment
What are the ways of coming into contact with an allergen?
Ingestion
Inhalation
Skin Contact
Injection
When does urticaria appear?
Typically within 1 hour and lasts 2-6hrs
What is angioedema?
localised swelling of subcutaneous tissue or mucous membranes
non pitting oedema
not itchy (unless associated with urticaria)
What is anaphylaxis?
- pharyngeal or laryngeal -oedema
- bronchospasm
- tachypnoea
- circulation hypotension
- tachycardia
- mucosal changes
What investigations can be done for allergy?
- History
- Specific IgE (RAST)
- Skin prick or patch test
- Challenge test
- serum mast tryptase level
How can allergy be managed?
allergen avoidance
prevent effects of mast cell activation (anti-histamines)
anti-inflammatory agent (corticosteroids)
adrenaline autoinjector (for anaphylaxis)
block mast cell activation (mast cell stabilisers- sodium cromoglycate)
immunotherapy
medic alert bracelet
information and education
Name some non allergic reactions
Coeliacs
Eosinophilic gastroenteritis
Direct mast cell degranulation- morphine, aspirin, NSAIDS
toxic (scombroid fish toxins)
What type of allergy is allergic contact dermatitis?
Type IV
When does type IV allergy appear?
24-48hrs
Name some causes of IV allergy?
Direct skin contact
Airborne contact
Injection
What are the two phases of allergy pathophysiology?
Sensitization phase
Elicitation phase
What are the four steps of the afferent phase?
- Haptens penetrate the epidermis and are taken up by epidermal cells, including skin.
- Dendritic cells (DCs) migrate to the draining lymph nodes,
- where they present haptenated peptides to both CD8+ effector T cells and downregulatory CD4+ T cells.
- Specific T-cell precursors clonally expand in draining lymph nodes, recirculate via the blood, and migrate to tissues, including the skin.
What are the steps of the elicitation phase?
- When the same hapten is applied to the skin, it is taken up by epidermal cells, including skin DCs and keratinocytes,
- which present haptenated peptides to specific T cells.
- Activation of CD8+ cytotoxic T lymphocytes induces apoptosis of keratinocytes and production of cytokines and chemokines by skin resident cells.
- This leads to the recruitment of leukocytes from the blood to the skin. CD4+ T cells may block activation/expansion of CD8+ effectors in lymph nodes during sensitization and in the skin during the elicitation phase of contact hypersensitivity
