This document discusses hypersensitivity reactions, specifically Type I or immediate hypersensitivity reactions which cause allergic responses. It defines a hypersensitivity reaction as an abnormal immune response to antigens. Type I reactions are mediated by IgE antibodies and include common allergies. The mechanisms involve IgE binding to mast cells and basophils, antigen cross-linking of IgE, and subsequent release of inflammatory mediators. Diagnosis methods include skin tests and IgE measurement. Treatments focus on avoidance, desensitization, and drugs that reduce inflammation or inhibit mediator effects.

1. HYPERSENSITIVITY REACTIONS
MODULE - IV
IMMUNOLOGY AND IMMUNOTECHNOLOGY
M.SC. MICROBIOLOGY / BIOTECHNOLOGY 2022-24
PRESENTED BY: ABHILASH JEAS GEORGE
M.SC. MICROBIOLOGY
1

2. What is a hypersensitivity reaction?
• It is an abnormal physiological condition in
which there is an undesirable and adverse
immune response to antigen.
• It is caused by many types of particles and
substances from the external environment
or from within the body that are recognized
by the immune cells as antigens.
• The immune reactions are usually referred
to as an over-reaction of the immune
system and they are often damaging and
uncomfortable.
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Fig. Young girl sneezing to flowers
Source: Getty images

3. Types and Classification
Two broad types: Immediate & Delayed
1. Immediate hypersensitivity reactions result in symptoms that
manifest themselves within very short time periods after the
immune stimulus (within 24 hours).
2. Delayed-type hypersensitivity (DTH) usually occurs more than 12
hours after exposure to the allergen, with a maximal reaction time
between 48 and 72 hours.
In general, immediate hypersensitivity reactions result from antibody-
antigen reactions, whereas DTH is caused by T-cell reactions.
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4. Types and Classification
contd.....
More specific classification given by Gell and Coombs → Type I – IV
1. Type I hypersensitivity reactions: mediated by IgE antibodies, and include
many of the most common allergies to respiratory allergens, such as pollen and
dust mites.
2. Type II hypersensitivity reactions: due to binding of IgG or IgM to the surface
of host cells, which are then destroyed by complement- or cell-mediated
mechanisms.
3. Type III hypersensitivity reactions: antigen-antibody complexes deposited on
host cells induce complement fixation and an ensuing inflammatory response.
4. Type IV hypersensitivity reactions: result from inappropriate T-cell activation.
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5. 16-09-2023 5
Fig. The four types of
hypersensitivity reactions.
Source: Kuby Immunology,
6th ed.

6. Type I Hypersensitivity reaction (allergies)
Type I reaction: Initiated by the interaction
between an IgE antibody and a multivalent
antigen.
Healthy individuals: Generate IgE antibodies
only in response to parasitic infections.
Atopic people: Predisposed to generate IgE
antibodies against common environmental
antigens.
Most allergens are either protein or
glycoprotein in nature, with multiple antigenic
sites, or epitopes, per molecule.
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Fig. Common allergens
Source: Kuby Immunology, 6th ed.

7. Biological Basis of allergen activity
1. Many allergens have intrinsic enzymatic activity (protease, etc.) that
affects the immune response
• capable of disrupting the integrity of epithelial cell junctions
• cleave and stimulate protease-activated receptors on the surfaces of
immune cells, enhancing inflammation.
2. Many allergens contain potential PAMPS
• capable of interacting with receptors of the innate immune system, and
initiating a cascade of responses leading to an allergic response.
3. Many allergens enter the host via mucosal tissues at very low
concentrations
• tend to predispose the individual to generate TH2 responses, leading to B-
cell secretion of IgE.
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Type
I
Hypersensitivity
reaction
(allergies)

8. General Mechanism
Source: Kuby Immunology, 6th ed.
16-09-2023 8
Type
I
Hypersensitivity
reaction
(allergies)

9. General Mechanism
• Exposure to an allergen activates TH2 cells that stimulate B cells to
form IgE-secreting plasma cells.
• The secreted IgE molecules bind to IgE-specific Fc receptors (high
affinity receptor FcεRI) on mast cells and blood basophils. (Many
molecules of IgE with various specificities can bind to the FcεRI.)
• Second exposure to the allergen leads to cross-linking of the bound
IgE, triggering the release of pharmacologically active mediators
(vasoactive amines) from mast cells and basophils.
• The mediators cause smooth muscle contraction, increased
vascular permeability, and vasodilation.
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Type
I
Hypersensitivity
reaction
(allergies)

10. Low-affinity and High-affinity IgE receptors
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Type
I
Hypersensitivity
reaction
(allergies)
(a) High-affinity IgE receptors:
Mast cells and basophils
constitutively express high levels of
the high-affinity IgE receptor, FcεRI,
which binds IgE with an
exceptionally high-affinity constant
of 1010 M-1. This affinity helps
overcome the difficulties associated
with responding to an exceptionally
low concentration of IgE in the
serum.
(b) Low-affinity IgE receptors:
Designated as FcεRII, or CD23, has
a much lower affinity for IgE. CD23a
is found on activated B cells,
whereas CD23b is induced on
various cell types by the cytokine
IL-4. Can bind CD21 also.

11. Signalling pathways initiated by IgE cross-linking
By cross-linking Fcε receptors, IgE initiates signals
that lead to mast cell degranulation, cytokine
expression, and leukotriene and prostaglandin
generation.
i. Briefly, cross-linking of FcεRI activates tyrosine
kinases, including Lyn, which phosphorylate
adaptor molecules, leading to activation of
multiple kinases, including protein kinase C
(PKC) and various mitogen-activated protein
kinases (MAPKs).
ii. activate transcription factors (e.g., NFκB) that
regulate cytokine production.
iii. also activate lipases, including phospholipase D
(PLD), which regulates degranulation, and
phospholipase A (PLA), which regulates the
metabolism of the leukotriene and
prostaglandin precursor arachidonic acid.
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Type
I
Hypersensitivity
reaction
(allergies)

12. Mast cell activity
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Type
I
Hypersensitivity
reaction
(allergies)

13. Principal mediator molecules
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Type
I
Hypersensitivity
reaction
(allergies)
Primary mediators
are preformed and
stored in granules
prior to cell
activation, whereas
secondary mediators
are either synthesized
after target-cell
activation or released
by the breakdown of
membrane
phospholipids during
the degranulation
process.

14. Categories of Type I hypersensitivity
reactions
• The clinical manifestations of type I reactions can range from life-
threatening conditions, such as systemic anaphylaxis and severe asthma,
to localized reactions, such as hay fever and eczema.
• Systemic Anaphylaxis: A shock-like and often fatal state that occurs
within minutes of exposure to an allergen. Usually initiated by an allergen
(such as anti-toxins, sea-foods, etc.,) introduced directly into the
bloodstream or absorbed from the gut or skin. Occurs due to rapid
antibody-mediated degranulation of mast cells and the systemic effects
of their contents.
• Localized reactions: In localized hypersensitivity reactions (atopy), the
pathology is limited to a specific target tissue or organ, and often occurs
at the epithelial surfaces first exposed to allergens.
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Type
I
Hypersensitivity
reaction
(allergies)

15. Early and late responses in a Type I HR
Early response: Occurs within minutes of allergen exposure; results from the
release of histamine, leukotrienes, and prostaglandins from local mast cells.
Late response: Hours after the early phase, cytokines released from mast cells,
particularly TNF-α and IL-1, increase the expression of cell adhesion molecules
on venular endothelial cells, thus facilitating the influx of neutrophils,
eosinophils, and TH2 cells that characterizes this phase of the response.
• Eosinophil chemotactic factor, released by mast cells during the initial reaction,
attracts large numbers of eosinophils to the affected site.
• Cytokines released at the site, including IL-3, IL-5, and GM-CSF, contribute to
the growth and differentiation of these cells, which are then activated by
binding of antibody-coated antigen.
• This leads to degranulation and further release of inflammatory mediators that
contribute to the extensive tissue damage typical of the late-phase reaction.
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Type
I
Hypersensitivity
reaction
(allergies)

16. Diagnosis
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Type
I
Hypersensitivity
reaction
(allergies)
Skin Test: Relatively safe; allows screening of a wide range of antigens
at once.
• Small amounts of potential allergens are introduced at specific skin
sites, either by intradermal injection or by dropping onto a site of a
superficial scratch.
• Thirty minutes later, the sites are re-examined.
• Redness and swelling (the result of local mast cell degranulation)
indicate an allergic response.
ELISA: Less commonly, physicians may elect to measure the serum
levels of either total or allergen-specific IgE using ELISA or Western
blot technologies.

17. Treatment
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Type
I
Hypersensitivity
reaction
(allergies)
1. First and Foremost, avoid the allergenic agents.
2. Hyposensitization: Treating allergic patients with repeated exposure (via
ingestion or injection) to increasing doses of allergens.
• Working principle: Repeated injection or ingestion of low doses of antigen
may lead to immune tolerance via the induction of Treg that quell the
immune response to the allergen.
• Alternatively or in addition, it may induce the generation of IgG antibodies
(specifically IgG4), which either compete with IgE for binding to antigen or induce
co-clustering of FcεRI with inhibitory FcγRII receptors. This inhibits basophil and
mast cell activity, reducing symptoms (desensitization).
**For Asthma: Drugs that enhance production of the second messenger
cAMP help to counter the bronchoconstriction of asthma and the
degranulation of mast cells. Examples include Epinephrine, Theophylline, etc.

18. Treatment
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Type
I
Hypersensitivity
reaction
(allergies)
3. Antihistamines, Leukotriene Antagonists, and Inhalation Corticosteroids:
• Antihistamines inhibit histamine activity by binding and blocking histamine
receptors on target cells. E.g., chlorpheniramine, used in controlling the
symptoms of allergic rhinitis.
• Leukotriene antagonists, specifically montelukast, have also been used to
treat type I hypersensitivities.
• Inhalation therapy with low-dose corticosteroids reduces inflammation by
inhibiting innate immune cell activity.
4. Immunotherapeutics: Therapeutic anti-IgE antibodies have been
developed; e.g., Omalizumab, approved by the FDA; binds the Fc region of
IgE and interferes with IgE-FcεR interactions.

19. Type II Hypersensitivity reaction
Type II hypersensitivity reactions involve antibody-mediated destruction of cells by
immunoglobulins of heavy chain classes other than IgE.
Antibody bound to a cell-surface antigen can induce death of the antibody-bound cell by
three distinct mechanisms:
• First, certain immunoglobulin subclasses can activate the complement system, creating
pores in the membrane of a foreign cell.
• Secondly, antibodies can mediate cell destruction by antibody dependent cell-mediated
cytotoxicity (ADCC), in which cytotoxic cells bearing Fc receptors bind to the Fc region of
antibodies on target cells and promote killing of the cells.
• Finally, antibody bound to a foreign cell also can serve as an opsonin, enabling
phagocytic cells with Fc or C3b receptors to bind and phagocytose the antibody-coated
cell.
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20. 16-09-2023 20
Fig. Three
mechanisms of
Type II
hypersensitivity
reactions
Source: Katie Ris-
Vicari
Type
II
Hypersensitivity
reactions

21. Complement mediated lysis of cells
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Type
II
Hypersensitivity
reactions
• Complement system is a system of lytic enzymes which are usually
inactive in blood.
• Enzymes of complement system are activated by antigen-antibody
complex.
• When antibody binds to antigen (microorganism or RBC) they form Ag-ab
complex.
• Ag-ab complex can activate complement system by three different
mechanisms - classical pathway, alternate pathway and lectin pathway.
• Activated complement proceeds in cascade mechanism.
• When complement is activated on the surface of cell (RBC) it causes lysis
of cell.

22. Antibody-dependent Cellular Cytotoxicity
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Type
II
Hypersensitivity
reactions
• Antibody binds with antigen by its Fab
portion. However Fc region of antibody has
receptor on cytotoxic cells. So, antibody
cross link target cell (microorganism or RBC)
with cytotoxic cells and promote killing.
• Most cytotoxic cells contain storage of
hydrolytic and digestive enzymes. These
enzymes are released on the surface of
target cell (MOs or RB or target cell), killing
them.
• Here, antibody itself does not kill or destroy
cell but rather mediate killing by presenting
antigen to cytotoxic cell. Similarly cytotoxic
cell depends upon antibody to bind antigen.
So this mechanism is known as Antibody
dependent cell mediated cytotoxicity.

23. Opsonisation
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Type
II
Hypersensitivity
reactions
• When antigen enters into host body, antibodies are produced.
• Antibody binds to antigen through Fab region. Fc region of antibody remains free.
• Phagocytic cells such as Neutrophils, macrophages and monocytes have receptors that can bind to Fc region of
antibody. The receptor is known as FcR.
• In this case antibody molecule directly cross links antigen (Micro-organism or RBC or target cell) with
phagocytic cells. This cross-linkage activates phagocytic cells and increases the rate of phagocytosis.
• This increased rate of phagocytosis by binding of antibody to antigen is called Opsonization.

24. Transfusion reactions
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Type
II
Hypersensitivity
reactions
Fig. ABO (ABH) blood
groups
Source: Kuby Immunology,
6th ed.

25. Example of TIIHR: Transfusion reactions
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Type
II
Hypersensitivity
reactions
• Human RBCs contains A and/or B antigen as major antigen. Other minor
antigens such as Rh, Kell, Duffy etc are also present.
• Antibodies to ABO antigen are called ‘isohemagglutinin’ and are usually of IgM
class whereas, antibodies to other minor antigen are of IgG class.
• An individual with blood group A recognizes B antigen like epitope (blood
group B) as foreign and produces isohemagglutinin. The same individual does
not produce antibodies to A antigen as it is similar to self antigen, so that state
of tolerance exists.
• If individual with blood group A is transfused with blood containing B antigen
then transfusion reaction occurs in which anti-B isohemagglutinin (antibodies)
binds with B-blood cells and mediate destruction of transfused RBC by
complement activation.

26. Example of TIIHR: Transfusion reactions
(continued….)
16-09-2023 26
Type
II
Hypersensitivity
reactions
• Since lysis of RBC occurs in intravascular space, free
hemoglobin appears in urine. Hemoglobin may be converted
into billirubin which is highly toxic to tissues.
• In case of minor antigen (Rh) mismatched, delayed reaction
occurs and in this case transfused RBCs are lysed by
opsonization (IgG antibody) and no free hemoglobin appears
in urine.

27. Example of TIIHR: Erythroblastosis fetalis
(Hemolytic disease of new born)
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Type
II
Hypersensitivity
reactions
Fig. Destruction of Rh
positive red blood cells
during erythroblastosis
fetalis of the newborn
Source: Kuby Immunology,
6th ed.

28. Example of TIIHR: Erythroblastosis fetalis
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Type
II
Hypersensitivity
reactions
• Hemolytic disease of newborn develops when maternal IgG antibodies specific for fetal
blood group crosses placenta and destroy fetal RBCs.
• The consequences of such transfer of antibody can be minor, serious or lethal to fetus.
• Serious hemolytic diseases of new born develops when Rh –ve mother conceives Rh +ve
fetus, which causes erythroblastosis fetalis.
• During pregnancy, fetal RBCs are separated from mother’s circulation by a layer of cell in
placenta called trophoblast.
• During the 1st pregnancy with Rh +ve fetus, mother’s circulation is not exposed to enough
fetal RBC to activate Rh-specific B cells for antibody production.
• At the time of delivery, large amount of fetal umbilical cord blood enters mother’s
circulation. These fetal blood activates mother Rh-specific B cells resulting in production
of plasma cell and memory cell. The plasma cell produce IgM antibodies which binds and
destroy fetal RBCs from mother’s circulation, but the memory cell remains which threats
any subsequent pregnancy with Rh +ve fetus.

29. Example of TIIHR: Erythroblastosis fetalis
(continued….)
16-09-2023 29
Type
II
Hypersensitivity
reactions
• Activation of memory cells in subsequent pregnancy with Rh +ve fetus
causes production of IgG antibodies which can cross placenta and destroy
fetal RBCs.
• Mild to severe anemia develops in fetus and sometime fetal.
• The conversion of hemoglobin to billirubin produces additional threat to
new born because billirubin may accumulate in brain and damage it.
• This hemolytic disease of new born can be prevented by injecting
preformed antibodies against Rh antigen to mother at around 28 weeks of
pregnancy and within 24-48 hours of 1st delivery.
• The antibodies are marketed as Rhogam. These antibodies bind to RBCs of
fetus in mother’s circulation and clear before B-cell activation.

30. Example of TIIHR:
Drug induced hemolytic anemia
16-09-2023 30
Type
II
Hypersensitivity
reactions
• Certain antibiotics (e.g., penicillin, cephalosporins, and streptomycin), as
well as other well-known drugs (including ibuprofen and naproxen), can
adsorb nonspecifically to proteins on red blood cell membranes, forming
a drug-protein complex.
• In some patients, such drug-protein complexes induce formation of
antibodies.
• These antibodies then bind to the adsorbed drug on red blood cells,
inducing complement-mediated lysis and thus progressive anemia.
• When the drug is withdrawn, the hemolytic anemia disappears.

31. Type III (Immune-complex mediated)
Hypersensitivity reaction
The reaction of antibody with antigen generates
immune complex.
In most of the cases, these immune complexes are
removed from blood circulation by various
mechanisms.
Type III hypersensitivity reaction is characterized by
deposition of immune complexes on various
tissues such as wall of blood vessels, glomerular
basement membrane of kidney, synovial membrane
of joints and choroid plexus of brain.
Deposition of immune complexes initiates reaction
resulting in damage of surrounding tissue and
cause inflammation.
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32. Mechanism of TIIIHR
16-09-2023 32
Type
III
Hypersensitivity
reactions
Type III hypersensitivity
reaction develops when
immune complex activates
C3a and C5a components
of complement system.
C3a and C5a are
lymphotoxin that causes
localized mast cell
degranulation.
Degranulation of mast cell
releases histamine which
increases vascular
permeability of blood
capillaries. This facilitates
deposition of immune
complexes on wall of blood
vessel.

33. Mechanism of TIIIHR (continued…..)
16-09-2023 33
Type
III
Hypersensitivity
reactions
• C5a, C3a and C5b67 also acts as chemotatic factors for neutrophils, So it
attracts neutrophils at the site of immune complex deposition.
• C3b acts as opsonin by binding with immune complex.
• Neutrophil binds to C3b coated immune complex by means of type I
complement receptor which is specific for C3b.
• The neutrophils attempt to phagocytose the immune complex but
phagocytosis is not possible because immune complexes are deposited
on basement membrane, so the neutrophil releases lytic enzymes to
destroy immune complex.
• The lytic enzymes cause tissue damage surrounding of immune complex
deposits, resulting hypersensitivity reaction. Furthermore, complement
proteins can also contribute to tissue destruction.

34. Examples of diseases resulting from TIIIHR
16-09-2023 34
Type
III
Hypersensitivity
reactions

35. Arthus reaction
(Localized Type III Hypersensitivity Reactions)
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Type
III
Hypersensitivity
reactions
• Acute Arthus reaction is an example of localized Type III
hypersensitivity reaction.
• When antigen is injected or enters intradermally or
subcutaneously, they bind with antibody to form
localized immune complexes which mediate acute
Arthus reaction (inflammatory reaction at site of
injection) within 4 to 8 hours.
• As the reaction develops, localized tissue damage and
vascular damage results in accumulation of fluids
(edema) and RBCs (erythema) at the site of antigen
entry.
• The severity of reaction can vary from mild swelling and
redness to tissue necrosis. Fig. An acute Arthus reaction
Source: Kuby Immunology, 6th ed.

36. Type IV (Delayed-type) Hypersensitivity
reaction
• Type IV hypersensitivity reaction is also known as delayed type hypersensitivity (DTH).
• When some subpopulation of activated TH cells encounters certain antigen, they secrete
cytokines that induce a localized inflammatory reaction called delayed type
hypersensitivity (DTH). The reaction is characterized by influxes of non-specific
inflammatory cells particularly macrophages.
• DTH occurs slowly and reaches a peak level within 48-72 hours after 2nd encounter of
antigen.
• In DTH tissue damage is mediated by TH cells and macrophages but not by antibodies.
• Although DTH causes some tissue damage, it is very important defense mechanism
against intracellular microorganisms such as Mycobacterium tuberculosis, Mycobacterium
leprae, Brucella spp., etc.
• Occurs in two phases: Sensitization and Effector phase.
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37. Sensitization Phase
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Type
IV
(DT)
Hypersensitivity
reactions
• DTH begins with initial sensitization by primary contact with antigen. At
first antigen is processed and presented by antigen presenting cells (APCs)
to CD4+ T-cell.
• CD4+ T-cells are activated to form TH cells.
• During this TH cells are clonally expanded by binding with MHC-II molecule
carrying antigen by appropriate APCs. Varieties of APCs have been shown
to be involved in activation of DTH response including Langerhans cell
and macrophages.
• CD4+ T-cell are the primary cell activated during sensitizing phase of DTH
response. However in some cases CD8+ cells are also found to induce DTH
response.

38. Sensitization Phase
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Type
IV
(DT)
Hypersensitivity
reactions

39. Effector Phase
16-09-2023 39
Type
IV
(DT)
Hypersensitivity
reactions
• A subsequent or second time exposure to antigen induces the effector phase.
• In this phase, TH1 cell secretes varieties of cytokines that recruits and activates
macrophages and other non-specific inflammatory cells to the site of antigen
injection.
• Macrophages are the principle effector of DTH response. The activated
macrophages exhibit increased level of phagocytosis and increased ability to kill the
antigen (microorganisms) by various cytotoxic lytic enzymes.
• Activated macrophages releases lytic enzymes that damage surrounding tissues
and intracellular bacteria.
• The influx and activation of macrophages in DTH is important in host defense against
intracellular bacteria and parasite, where circulating antibodies cannot reach them.
• Increased phagocytic activity and build up lytic enzyme from macrophages in the
area of infection leads to non-specific destruction of tissues and intracellular
pathogens.

40. Effector Phase
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Type
IV
(DT)
Hypersensitivity
reactions

41. Detection of DTH reaction: Tuberculin Skin Test
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Type
IV
(DT)
Hypersensitivity
reactions
• The presence of a DTH reaction can be measured experimentally by injecting
antigen intradermally into an animal and observing whether a characteristic
skin lesion develops days later at the injection site.
• A positive skin-test reaction indicates that the individual has a population of
sensitized TH1 cells specific for the test antigen.
• For example, to determine whether an individual has been exposed to M.
tuberculosis, PPD, a protein derived from the cell wall of this mycobacterium, is
injected intradermally.
• Development of a red, slightly swollen, firm lesion at the site between 48 and 72
hours later indicates previous exposure.
Clinical examples of T(IV)HSR: Coeliac disease, Giant-cell arteritis, etc.

42. 16-09-2023 42
Summary:
Hypersensitivity
reactions