Hypersensitivity Reactions



Antigen-antibody reaction can be caused by one of two basic mechanisms:

  1. Allergic reactions with immediate response (Types I,II,III ) Humoral immune response stimulated by the sequence : B-Lymphocytes → plasma cells → immunoglobulins.
  2. Allergic reactions with delayed response ( Type IV ) Cell – mediated immune response stimulated by T – lymphocytes.

REACTION TYPE ANTIGEN ANTIBODY
Anaphylactic type

Type I
Dust, feathers,Pollen, foodstuffs,

Drugs
Anaphylactic antibodies of class

IgE
Cytotoxic Type

(Type II)
Cell membrane fragments or antigens

(haptens) deposited on cell Cell
Cytotoxic antibodies of classes IgG ,IgM,IgA
Arthus reaction

(Type III)
Foreign serum drugs, antigens

from streptococci and tumors
Precipitating antibodies of class

IgG plus complement
Cell-mediated defense ( Type IV )
1.Eczematous type
2.Tuberculin type
Chemicals and noxious substances

of different kinds
T-lymphocytes

Immunological Reactions and Effects

Reaction Mechanism Effects Clinical manifestation
The reaction of IgE with the antigens
causes degranulation of the mast cells,
followed by release of histamine
serotonin, bradykinin and SRS-A
Vaso-dilation,
increase of vascular
permeability,
contraction of smooth muscle
Urticaria, angioedema,
anaphylactic shock,
Bronchial asthma,
Allergic rhinitis
Cellular antigens react with circulating
antibodies, or cellular antibodies react
with circulating antigens. Complement
becomes attached to the antigen-antibody
complex, leading to lysis of the target cell.
Cytolysis Thrombocytopenia agranulocytosis
hemolytic anemia, autoimmune
disease
Deposition of antigen-antibody
Complexes in capillary walls.
Activation Of the complement system.
Leukotaxis And phagocytosis of the
immune Complexes by the leukocytes.
Lysis of The phagocytosing cells and
destruction Of the cell walls by
lysosomal enzymes
Inflammation,
necrosis
Arthus, phenomenon,
serum sickness ,
drug rash toxic
epidermal necrolysis,
erythema multiforum
erythema nolosum
As a result of the lymphocyte-antigen
reaction humoral factors ( lymphokines )
are released Allergens invade the
epidermis (contact Dermatitis) Allergens
invade the dermis (tuberculin Type )
Vasodilation edema,
monolympho cytic infiltration
Allergiccontact dermatitis,
allergic photo-dermatitis,
allergic Exanthems,id reactions,
Transplant rejection

Hypersensitivity Reactions Of Irritants In Epidermis

In epidermal hypersensitivity reactions, the irritants directly the affect epidermis

Irritant and allergic
contact dermatitis
Direct injury to skin caused by toxic chemicals
(irritant contact dermatitis) or by substances which,
though not primarily toxic, affect persons
previously sensitized to the contact allergen.
Intertrigo Toxic injury to skin caused by sweat and metabolic
Products of saprophytic bacteria.
sunburns Skin damage caused by exposure to sun light
phototoxic and photo-allergic
dermatities
Skin damage caused by psoralen (furocoumarin),
and other photoxic or photo-allergic agents and
subsequent exposure to sunlight .
Nummular (bacterial ) dermatities Skin damage due to bacterial antigens or bacterial toxins
Seborrheic dermatitis Skin damage due to quantitative and qualitative
changes in sebum and sweat secretion.
Atopic dermatitis Skin damage due to constitutional factors
( chronic abnormal vegetative response).

Hypersensitivity reactions of the skin with Vascular involvement

In this type of hypersensitivity reaction the irritant Irritant affects both the cutis and the blood vessels.

Urticaria
angioedima
Edematization of the subcutaneous tissue due to
tissue due to allergic antigen –antibody reaction,
mechanical irritation or toxic factors.
Anaphylactic shock Massive disturbance of circulatory function due to
generalized anaphylactic hypersensitivity reaction
Of the immediate type ( Type ) .
Erythema multiforme Reaction of diverse etiology which may be caused
by bacterial or viral antigens or be drug-induced.
Results in edematious saturation of the tissue and
characteristic lesions.
Toxic and allergic Exanthems Vascular hypersensitivity reaction caused by
action of various toxic or allergenic substance
( especially drugs ) ; commonly manifests as a
polymorphic eruption .
Allergic cutaneous
Vasculitis
An allergic-hyperergic reaction of the small
vessels of the dermis associated with drug
therapy or streptogenic infections .
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