The document summarizes the four main types of hypersensitivity reactions: Type I are immediate and IgE-mediated, such as allergic reactions. Type II involve antibodies binding to antigens on cells, marking them for destruction. Type III occur when antigen-antibody complexes deposit in tissues, activating complement and attracting inflammatory cells. Type IV are delayed cell-mediated reactions involving T cells and cytokines. Skin tests can help identify the specific antigen responsible for types I-IV reactions.

1. HYPERSENSITIVITY
REACTIONS
DR. NIDHI PUGALIA

2. INTRODUCTION:
 Hypersensitivity reactions (HSR) are immune responses produced by the normal
immune system that are exaggerated against an antigen or allergen .
 Coombs and Gell classified hypersensitivity reactions into four types :
 TYPE I (ANAPHYLACTIC)
 TYPE II (ANTIGEN – ANTIBODY)
 TYPE III (IMMUNE-COMPLEX)
 TYPE IV (DELAYED)
IMMEDIATE HSR
(<24HRS)

3. TYPE 1 HSR (ANAPHYLACTIC REACTIONS)
 IGE MEDIATED
 CELLS INVOLED : MAST Cells
 BASOPHILS
 Mast cells and basophils bind the IgE through high-affinity IgE receptors
On re-exposure, the antigen causes cross-linking of IgE already present on mast cells
degranulation and release of vasoactive mediators and proinflammatory cytokines
These mediators are responsible for the clinical manifestations of type I hypersensitivity.

5. EXAMPLES OF TYPE I HSR :
 Inhalants :
 pollen derived from the flowers of various trees
 spores or fragments of fungi (molds)
 fragments of household insects and mites
 hair, feathers , dander from animals and birds
 Ingestants :
 mostly foods or orally ingested drugs.
 The drugs that commonly cause type I reactions include aspirins and penicillins
 Physical agents :
 Sunlight, sweating, cold stimuli, friction, pressure.

6. TESTS FOR TYPE 1 HSR
The tests commonly employed to detect type I hypersensitivity
include:
 Prick test
 Intradermal test for immediate hypersensitivity
 Radioallergosorbent test (RAST)
 Provocation tests

7. Prick Test and Intradermal Test for
Immediate Hypersensitivity
• Skin is pricked with a hypodermic needle
• A drop of the antigen placed on the skin.
• The tests are usually done on the forearm,
upper arm or the back of the individual

8.  Normal saline is injected into an additional area to serve as a
control so that the wheal and erythema produced by the antigens
be compared with the reaction produced at the control site.
 The readings are taken at the end of 20 minutes after the injection/
prick.
 MEASUREMENT AND INTERPRETATION: Any reaction that is more
than twice the reaction at the control site can be considered
significant, thereby being allergic to the substance.

9. Radioallergosorbent Test (RAST)
 This test is meant to measure
circulating IgE antibodies specifically
reactive against an antigen.
Patient’s serum
react with
ready-made antigen-impregnated
paper discs.

10.  If the serum contains specific IgE antibodies
it will be bound to the antigen
 non-specific IgE antibodies
will get washed off during the subsequent rinsing.
 The amount of specific IgE fixed on antigen discs can be measured by
the addition of radioactively labelled antiserum against IgE.

11. Provocation Tests
 We can perform provocation tests by introducing the
suspected allergen into the site (nose, lungs , eye) and
measure to see if we can provoke an allergic response.
 Various techniques : Provocation with Cold
Provocation with Cholinergic Agents
Provocation for Dermographism
Provocation with Drugs
 It is also necessary that the patient should be completely
free from allergic symptoms and should not be taking
antiallergic drugs before provocation.

12. TYPE II (ANTIGEN – ANTIBODY HSR)
 The antibodies involved : IgG or IgM class.
 They circulate in the blood and permeate into various tissue fluids
reach the target antigen
Attachment of the antibody molecule to the corresponding antigen
(ANITGEN- ANTIBODY COMPLEX)
lysis and destruction of the cells.
 This may be achieved by :
 antibody-dependent cell-mediated cytotoxicity (ADCC)
 NK cells
 activation of the full cascade of complement components.

13. Three mechanisms have been described for type II HSR
 1. Opsonization and complement, and Fc receptor-mediated phagocytosis:
 Antigen presentation on the cell surface
complement activation
Immunoglobulin production.
C3b, C4b and IgG opsonize cells like RBCs, platelets and WBCs
Phagocytosis by macrophages.
Complement activation
formation of membrane attack complex
osmotic lysis of the cells.
 Examples: transfusion reactions, erythroblastosis fetalis, autoimmune haemolytic anaemia,
agranulocytosis, thrombocytopenia

14.  2. Complement and Fc receptor-mediated inflammation:
 The antigen is usually present in the extracellular matrix where the
antibody gets deposited.
complement activation and recruitment of neutrophils and monocytes
inflammation.
 Example: glomerulonephritis, ANCA associated vasculitis.

15.  3. Antibody-mediated cellular dysfunction:
The antibodies are directed against cell-surface receptors
Which impair ,deactivate or stimulate it without causing inflammation.
 Examples: myasthenia gravis, pemphigus vulgaris and Grave’s disease

16. Type II
HSR

17. TESTS
 The tests usually employed for type 2 HSR:
 (1) precipitation tests
 (2) agglutination and flocculation tests
 (3) complement fixation tests
 (4) Immunofluorescence test

18. Precipitation Tests
 Antibodies that form
precipitates with their
corresponding antigens can be
detected
Principle:
patient’s serum
react with
the antigen in some suitable
medium
precipitation

19. Agglutination and Flocculation Tests
 Some antigens can be fixed on to the
surface of erythrocytes or other inert
particles such as latex or bentonite
react with
patient’s serum
Ag-Ab combination results in
Clumping of erythrocytes
(agglutination) or other particles to
form floccules (flocculation)

20. Complement Fixation Test
 Used when antigen-antibody combination requires the participation of
complement.
Antigen
patient’s serum containing antibodies in the presence of a known amount of
complement obtained from guinea pig serum.
 Utilization of the complement is checked by an indicator system that
consists of sheep RBCs and an antiserum raised in rabbits against sheep
RBCs.
reacts with

21. INTERPRETATION:
Haemolysis of the sheep RBCs
indicates that the complement was available for the indicator system
therefore the patient’s serum did not contain the antibodies.
• No haemolysis
• the complement would be consumed in the test system
• would not be available for the indicator system
• antibodies are present

23. Type III (COMPLEMENT MEDIATED HSR)
 Antibodies involved in these reactions are usually of the IgG class.
The circulating antigen-antibody complexes
get deposited in
walls of blood vessels or on membranes.
Activation of complement
leading to
release of leukotactic factors from the complement components
Attraction of polymorphs to the site of action.
The lysosomes (neutrophilic granules) of the polymorphs release their lysosomal enzymes
which damage the tissues to produce-
 vasculitis if the complexes were located on the blood vessels,
 glomerulonephritis or serositis if they were located on the renal or serous membranes.

24. TYPE III HSR
Examples:
• systemic lupus erythematosus,
• toxic epidermal necrolysis
• erythema nodosum (including
erythema nodosum leprosum)
• Fixed drug eruptions

25. TESTS :
 The tests employed to detect type III reactions are generally the same as those used for type II reactions
to detect the presence of antigen-specific antibodies.
 The immunologic nature of the manifestations can be confirmed by demonstrating the presence of
antigen-antibody-complement complexes in the tissues by immunofluorescence tests.
 Immunofluorescence tests:
 Two types:
 Direct immunofluorescence (DIF) test
Indirect immunofluorescence (IIF) test
IgG/IgM

26. PRINCIPLE: The DIF test is based on the fact that antibodies
directed against a tissue component are deposited on the tissue
which contains the antigen.
Microscopic section of a tissue containing antigen
made to react with
Antiserum against IgG or IgM + conjugated with a fluorescent
dye
the anti-IgG/IgM serum
will react with
antibodies fixed on the antigen in the tissue and the fluorescent
dye tagged to the antiserum
Emission of fluorescence when viewed under a microscope

27.  PRINCIPLE: The IIF test is used to detect whether the serum of the patient contains an
antibody reactive against a particular tissue antigen.
microscopic sections of the tissue containing antigen.
is made to react
The patient’s serum
If serum contains antibodies
it will react with
corresponding antigen in the tissue.
Deposition of these antibodies
detected by
antiserum containing antibodies to IgG or IgM
and conjugated with the fluorescent dye.
A positive fluorescence will indicate the tissue antigen against which antibodies are present
in the serum.

28. Type IV Reactions (DELAYED HSR)
 Major dermatological disease based on this type of hypersensitivity reaction is contact
dermatitis.
Antigens in contact dermatitis are chemical compounds with small molecular weights called
haptens
require
Langerhans cells to initiate sensitization.
 Type IV HSR is mediated by activated T lymphocytes
liberation of
lymphokines on encountering antigen.
Attraction of macrophages
formation of
pathological lesion.

29.  Cell-mediated or type IV hypersensitivity reaction is of two types:
 a) Delayed type hypersensitivity
 b) T cell mediated cytotoxicity.
 A) Delayed type hypersensitivity
Persistent antigen
(which has not been cleared off by the immune system)
differentiation of CD4+ cells into Th1 cells
after first exposure
get stored as memory Th1 cells.
On re-exposure
Th1 cells secrete IFN-γ
key factor for activation of
macrophages into epithelioid cells.
The epithelioid cells have lymphocytes and a zone of fibrosis
surrounding them
epithelioid cell granuloma.

30. b) T cell mediated
cytotoxicity
Sensitized cytotoxic CD8+
T lymphocytes
Cell lysis
by
Perforin or granzyme or
Fas-Fas L pathways.

31. TESTS:
 The tests usually undertaken for detecting type IV HSR include:
 Intradermal test for delayed hypersensitivity
 Patch test for contact dermatitis.
 Intradermal Test for Delayed Hypersensitivity
(IDDH)
 The antigens are prepared from the respective infective
agents
injected intradermally.
48-72 hours
A significantly indurated nodule
Delayed hypersensitivity to the antigen.

32. Patch Test
 Procedure:
 Application of specified concentrations of chemicals or the
prepared ready-made objects in suitable bases, on the normal-
looking skin
covering these areas with occlusive patches for 48 hours.
At the end of 48 hours
patches are removed
sites examined for evidence of an inflammatory reaction.
Dermatitis at the site of a patch test indicates positive test.

34. Common antigens used in patch testing

35. Complications of Patch Testing
 Severe reaction: very severe reactions to the allergen leading to exacerbation of the
patient’s eczema.
 Plaster reactions: A mild plaster reaction due to irritation by an occlusive zinc oxide
strapping may occur.
In a few cases severe eczematous reactions may occur due to reaction to colophony.
 Persistent positive reaction: A patch test may remain positive for longer than 1 month.
 Anaphylaxis
 Depigmentation, scars and keloids

36.  Active sensitization:. When a patch test site becomes positive 10–14
days later, active sensitization may have occurred. Such reactions
are common with dinitrochlorobenzene (DNCB), cobalt, p-
phenylene-diamine
 Focal flare: It means activation at the patch test site. It could be due
to active sensitization or due to activation of the patient’s eczema
leading to flare up of a positive patch test reaction that had
completely subsided.

37. SUMMARY:

39. REFERENCES:
ROOK’S
BOLOGNIA
IADVL
ROBBINS

40. THANK YOU.