Allergy is an acquired
specific alteration in the capacity of the host to react with an antigen .
It is an immunological reaction depending on antigen-antibody complexes.
The allergic reaction may
be immediate that occurs after exposure to the antigen or delayed taking
few hours or even days or weeks to appear.
Allergic skin diseases
constitute the main and bulk of skin dermatoses in children and adults .
function of the skin
The skin is an
environmental barrier which initiates an immune response to the penetrating
This will lead to:
Destruction of infective
removal of the potential toxins and the damaged tissue.
The skin contains many
elements of the immune system, which have been designated, the
skin-associated lymphoid tissue (SALT) or the skin immune system (SIS).
Human epidermis contains
Langerhans cells, and Keratinocytes which when stimulated will synthesize
several types of Cytokines that activate and induce proliferation of
lymphocytes and macrophages, which contribute, induce and enhance
The immune system
functions in the skin as in other body organs are either a non-specific,
(innate) or specific type, (adaptive).
non-specific immune system
The non-specific immune
mechanism is the first line of the body defense when it is exposed to an
antigen which it includes:
performed by the skin.
(the mucous, celia in the respiratory tract, sebaceous gland
Cellular: this is
performed by the phagocytes or the natural killer cells (NK).
specific immune system
This type has its role
when the non-specific immune system failes to take care of the offending
The specific immune
system includes the cellular and humoral components that
characteristically have an immunological mirror memory, where it can
enhance an immunological response on repeated exposure to the same
of the specific immune system
The specific immune
system produces the immuno-competent cells, which are the T cells
and B-cells .
T- cells modulate both
the cellular and humoral immune responses .
B - cells produce the
different antibody immunoglobulin groups; IgG, IgE, IgM, IgA and IgD .
Allergic reaction may be
related to an endogenous or exogenous factors .
Antigen: is a foreign
substance that can stimulate an immunological response.
This will lead to:
Formation of antibodies.
Alters the reactivity of
lymphocytes, which is known as cell mediated response.
characteristics of antigens
Antigens may have high or
low molecular weight.
molecular weight antigens
are composed of amino acids and peptides that can stimulate the
formation of antibodies.
molecular weight antigens
cannot stimulate the formation of antibodies unless combined with a
protein. These are known as haptens. These can stimulate humoral
antibodies or delayed-type (cell-mediated) sensitivity, which is
specific for the haptens.
Antigens that commonly
sensitize persons to anaphylactic or to delayed-type hypersensitivity and
these are known as allergens.
Antibody: is a
serum immunoglobulin that is formed in response to antigen stimulation.
Immunoglobulins are synthesized by plasma cells and are composed of
The basic antibody unit
is the immunoglobulin molecule (Ig). There are different immunoglobulins
mainly IgG that constitutes around 75 per cent of the immunoglobulins.
Immunoglobulins: IgG, IgE, IgA, IgD are found in varying amounts in the
external secretions, e.g. respiratory, intestinal tract.
Failure of any part of
the immune system may predispose to allergy, infection or malignancy.
is either an immediat which appears after exposure to a certain antigen or
may be delayed type.
allergic reactions (Type (1, 11,
111) : these reactions represent a humoral response, stimulating
B-lymphocytes, plasma cells that produce and activate different
allergic reaction (Type 1V): The
allergic reaction may take few hours, days or even weeks to appear from
the time of exposure to the antigen. This type is cell mediated by
hypersensitivity reactions according to Gell and Coombs classification
immediate, anaphylactic type (Type I)
The antigens in this type
may be foodstuff, pollens, drugs, dust, feathers or others.
The antibodies are
The antigen -antibody
reaction causes degranulation of mast cells which release different
mediators mainly histamine, serotenin, bradykinin and slow reacting
physiopathological changes are:
Increase of the
the smooth muscles.
manifestations of Type I reactions are: Urticaria, angioedema,
anaphylactic shock, bronchial asthma and allergic rhinitis.
Cytotoxic antibody (Type II)
The antigen in Type II
allergic reaction is haptens, which deposits on the cell wall or cell
membrane fragments, which can act as an antigen.
The antibodies are
cytotoxic antibodies of the classes IgG, IgA and IgM.
The allergic reaction
occurs as a result of reaction of the circulating antibodies or the
cellular antibodies with the circulating antigens, which lead to
attachment of complements with the antigen-antibody complex leading to
lyses of the target cells.
The effect of such
reaction is cytolysis.
manifestations of the cytotoxic reactions are: Thrombocytopenia, hemolytic
anemia, agranulocytosis and autoimmune diseases.
Antigen - antibody immune complex (Type
III), Arthus reaction.
The antigens in this type
may be drugs, foreign serum and other different antigens such as from
streptococci and tumors.
The antibodies of Type
III reaction are immunoglobulins IgG and complements.
The allergic reaction occurs as a result
of complex mechanism.
antigen-antibody complexes in the capillary walls.
the complement system.
phagocytosis of the immune complex by leukocytes.
Lyses of the
the cell walls by the lysosomal enzymes.
The effects of these
complex reactions are inflammation and necrosis of tissues.
clinical manifestations of type III allergic reactions are:
reactions, arthus phenomenon, serum sickness.
multiforme and erythema nodosum.
4. Delayed type cell
mediated (Type IV). The antigens are different types of chemicals and
The antibody is the
The allergic reaction is
due to lymphocyte-antigen reaction due to humoral factors released
(lymphokines).The effects of these reactions are: vasodilatation, edema,
and monolymphocytic infiltration.
manifestations are: allergic contact dermatitis, allergic photodermatitis,
allergic exanthemas, id eruptions, and transplacental rejection.
The manifestation of the
allergic reaction may be localized only to the skin , internal organs or
The immediate and delayed
type of hypersensitivity reactions are the most important in dermatitis
Occurs as a result of
degranulation of mast cells due to an antigen stimulation leading to
release of mediators as histamine, leukotrienes, platelet activating
factor, proteases and others. This reaction may elicit skin disorders such
as in early stages of atopic eczema, urticaria and angioedema.
Investigation of this
type includes: skin prick test, serum IgE and (RAST) test.
Repeated contact of a
specific antigen with a previously sensitized T-cells leads to stimulation
and the release of lymphokines. Lymphokines induce inflammatory reactions
and activate macrophages to release mediators.
This type of reaction
corresponds to the delayed type of contact dermatitis, photo-allergic
dermatitis and late stages of atopic dermatitis.
Cell mediated immune
reactions are common in the defense against fungal, viral, granulomatous
infections of the skin and mediating reactions to insect bite.
Patch tests and
lymphocyte transformation may help to detect the sensitizing allergen.
Skin tests may be of
value in detecting the sensitizing antigen.
Eczema is an inflammatory
skin reaction in response to an antigen stimulation. Eczema is Greek word
, which means "to boil out" or "to effervesce".
Baer describes eczema as
a pruritic papulovesicular reaction, which is associated in its acute
phase with erythema and edema, and in its chronic phase by thickening,
lichenifecation, scaling, and retaining some of its papulovesicular
more than thirty per cent of clinical dermatology.
There are different
classifications of eczema, which are sometimes confusing and therefore
most authors prefer the word ‘dermatitis‘.
In spite that all cases
of eczema is dermatitis, not all cases of dermatitis is considered as
eczema since dermatitis means skin inflammation.
Classification of eczema
is sometimes arbitrary depending on the causative triggering factors.
Eczema may be classified
into: endogenous, which is due to processes originating within the body or
exogenous eczema that is, mediated by exogenous factors.
The term endogenous
eczema implies that the eczematous condition is not due to exogenous or
external environmental factors, but is mediated by processes originating
within the body.
In some conditions,
however, there are both external and internal precipitating factors.
Metabolic eczema or
eczema associated with systemic diseases.
Eczematous drug eruption
This type of eczema is
due to different exogenous factors that include the following:
Photo allergic dermatitis
Unclassified types of eczema
Eczema can be also
classified according to the course and clinical picture in to:
Sub acute eczema
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