- Allergy is the response of the immune system to small or harmless substances like dust, pollens, smoke, etc.
- It’s in fact called hypersensitivity reactions.
- Hypersensitivity refers to undesirable reactions produced by the normal immune system.
◦ Allergy (immediate)
◦ cytotoxic, antibody-dependent
◦ Immune complex diseases
◦ Delayed type, cell-mediated
Figure: Hypersensitivity Type I, Type II, Type III, Type IV. Image Source: Wikipedia.
TYPE I (Anaphylactic) Hypersensitivity
- The allergen stimulates the induction of CD4+T cells. These T cells secrete cytokines that cause IgE production by plasma cells.
- The IgE molecule will bind to the Fc receptor on mast cell and basophils which in turn causes vasodilation increased vascular permeability and vascular spasm. this type may occur as a systemic or local reaction:
- systemic reactions: skin erythema, followed by respiratory difficulty due to bronchial constriction.
- local reactions: generally on the skin or mucosal surface at the site of Ag exposure. Allergy to penicillin, Aspergillus spores, rupture of Echinococcus cyst…
TYPE II Hypersensitivity
In this type Ab are formed against target Ag that are cell membrane components. Not really hypersensitivity, but cytotoxic reactions:
- Ab reacts with cell surface Ag leading to fixation of the complement system and then cell lysis. eg. red cells are the most common cells damaged by this mechanism>>>HEMOLYTIC ANEMIA NO MICROBES!
B) many cell types (macrophages, neutrophils, NK cells) cause lysis of target cell coated by IgG
- Poststreptococcal rheumatic fever: molecular mimicry: Antibodies produced against S. pyogenes cross-react with various tissue eg. heart, joints – inflammation
- Oncocerca worm infection may lead to blindness because of the cross-reaction of Ab produced against pathogens and proteins of the retina.
C) Antibody-mediated cellular dysfunction
- In some cases Ab is directed against cell surface receptor impairing the function but not cause cell injury>>>MYASTHENIA GRAVIS; Ab reacts with ACh receptors on the motor endplate.
TYPE III Hypersensitivity
- This type is mediated by ag-ab complexes which initiate an inflammatory reaction in the tissue. there are 2 patterns of immune-complex mediated injury:
A) SYSTEMIC DISEASE (serum sickness serum sickness type, SLE)
- This is because of a large excess of Ab and immune complexes are deposited at the site of injury especially within the vessel wall, the subsequent events will result in necrotizing vasculitides and accumulation of neutrophils>>>SLE
B) LOCAL DISEASE (Arthus reaction)
- Arthus reaction: intraductal injection of antigens to a personalized person may lead to local intradermal Ab – Ag complex formation and local vasculitis, redness, swelling.
- Example: Repeated (booster) vaccination with diphtheria or tetanus rarely leads to local vasculitis.
- Poststreptococcal acute Poststreptococcal acute glomerulonephritis glomerulonephritis glomerulonephritis: Ab-ag complexes deposit in glomeruli HBV infection: HBsAg-Ab complexes may HBV also cause acute glomerulonephritis
TYPE IV Hypersensitivity
This is mediated by T-cells. There are 2 types that involve CD4/8+T Cells.
A) Acute (within 2-3 days)
- Tuberculin test, contact dermatitis: mediated by CD4+ T helper cells cd4+ cells recognize ag (tuberculin), this leads to the formation of sensitized cd4+ cells.
- Upon cutaneous injection into previously sensitized individual sensitized cd4+cells become activated and secrete cytokines.
- Tuberculin/Mantoux Tuberculin/Mantoux Mantoux test test test: intradermal injection of tuberculin = purified tuberculoprotein leads to swelling after 48- 72 h if the patient has been exposed to Mycobacterium tuberculosis previously.
- Important: BCG vaccination? Yes/No!
B) Chronic (> 1 week)
- Granuloma formation, graft rejection: mediated by cd8+ cytotoxic T cell.
- lymphocytes surrounding epitheloid cells lead to the formation of granuloma.
References and Sources
- 4% – http://semmelweis.hu/mikrobiologia/files/2014/05/hypersensitivity.pdf
- 2% – https://basicmedicalkey.com/streptococcus-enterococcus-and-similar-organisms/
- 1% – https://www.researchgate.net/publication/16984737_Immune_complexes_Characteristics_clinical_correlations_and_interpretive_approaches_in_the_clinical_laboratory
- 1% – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3597223/
- 1% – https://quizlet.com/228679925/week-vi-flash-cards/
- 1% – https://quizlet.com/12054435/cytokines-flash-cards/
- 1% – https://en.wikipedia.org/wiki/Red_blood_cell
- 1% – http://site.iugaza.edu.ps/asilmi/files/2017/09/14.pptx