L21- Hypersensitivity and allergy Flashcards
What is Type 1 hypersensitivity?
Immediate hypersensitivity mediated by IgE antibody found on mast cells. Allergen crosslinking causes stimulation
Tissue injury caused by mast cells and their mediators such as histamine, interleukins and TNF
What are examples of type 1 hypersensitivity?
- Asthma
- Allergic rhinitis
- Systemic anaphylaxis
What is Type 2 hypersensitivity?
Antibody mediated e.g IgM, IgG to cell surface extracellular matrix antigens
Tissue injury mediated by opsonisation, complement and phagocytosis
What are examples of type 2 hypersensitivity?
- Drug allergies
- Myasthenia gravis
- Graves disease
What is Type 3 hypersensitivity?
Immune complexes of circulating antigens and IgM or IgG
Complement and FCR mediated recruitment and activation of leukocytes mediates tissue injury
What are examples of type 3 hypersensitivity?
• Serum sickness
What is Type 4 hypersensitivity?
T cell mediated, CD4+ and CD8+ T cells
Tissue injury mediated by macrophage activation (cytokine mediated inflammation) and direct target cell lysis
What happens in systemic anaphylaxis?
- Caused commonly by drugs, venoms
- Route of entry is intravenous
- Response is oedema, vasodilation, tracheal occlusion, circulatory collapse and death
What happens in wheal and flare?
- Caused commonly by insect bites and allergy testing
- Route of entry is subcutaneous
- Response is local vasodilation and oedema
What happens in allergic rhinitis?
- Caused commonly by pollens and dust mites
- Route of entry is inhaled
- Response is oedema and irritation of nasal mucosa
What are the stages of anaphylaxis?
Stage 1- Generalised itching
Stage 2- Swelling away from sting and incontinence
Stage 3- Difficulty in breathing
Stage 4- Fall in blood pressure and loss of conciousness
What do mast cells release?
A. Histamines, serotonins (short-lived)- increase local blood flow and vascular permeability
B. Cytokines, leukotrienes (sustained)- smooth muscle contraction
C. Cytokines like IL4 and TNF-a cause sustained inflammatory signal
What is peripheral tolerance?
A state of tolerance acquired by mature lymphocytes in the peripheral tissues as opposed to central tolerance, which is acquired by immature lymphocytes during their development
Why is maintaining peripheral tolerance important?
- Self reactive lymphocytes can escape negative selection in thymus (central tolerance) due to low affinity to self MHC/antigen and the exclusive expression of some proteins in periphery
- These upon recognition of specific auto-antigens can cause tissue damage and autoimmune disease like RA. T1 diabetes
How is peripheral tolerance maintained?
- Deletion/Apoptosis/Anergy when lack of costimulation is present
- Immunological ignorance- no contact with self Ag
- Immune deviation- Th1 to Th2
- Active suppression by Tregs
