Hypersensitivity
immunology
What is hypersensitivity?
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the violent reaction of the immune system leading to severe
symptoms and even death in sensitised animal when it is reexposed to the same antigen for the second time. It is nothing
but allergy.
Extreme sensitivity to antigens.
Immune response is always directed towards the
protection of the host.
But in hypersensitivity the immune response becomes
injurious to the host.
Hence, the immune response becomes a destructive
process in hypersensitivity.
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In protective immune response, the antigen or bacteria or
virus is killed or neutralized.
But in hypersensitivity, the cells of the host are killed of
the host itself is damaged or killed.
Hypersensitivity is the changed reactivity of the immune
system. It is a beneficial protective system gone out of order.
Situation in which host immune response contribute to
tissue injury are collectively referred to as
hypersensitivity states.
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The factors causing hypersensitivity are called allergens.
In clinical terms, hypersensitivity is called allergy.
Factors causing hypersensitivity
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Drugs
Air borne particles
Infectious organisms
Food
Types of hypersensitivity
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Type I: Immediate hypersensitivity reactions.
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Type II: Antibody Dependent Cytotoxic Hypersensitivity
Reactions
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Type III: Immune-Complex mediated Hypersensitivity
Reactions.
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Type IV: Cell- Mediated Hypersensitivity Reactions
Type I: Immediate hypersensitivity reactions
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is mediated by IgE antibodies which have bound to
the Fc receptors of circulating basophils and tissue
mast cells, thereby sensitizing them.
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When antigen contacts the sensitized cells, an
immediate skin reaction occur leading to intense local
inflammation typified by diffuse infiltrations by
polymorphonuclear leukocytes,
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which results in a soft, swollen red skin reaction.
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The chemical basis of reaction is the release of histamine,
serotonin, and heparin by the sensitized basophils and
mast cells.
Besides skin reaction it also affects smooth muscles,
vessel walls characterized by the symptoms of irritation,
rashes, swelling, wheezing shock and occasionally death anaphylactic reactions
Eg.Allergic reactions to mosquitoes and wasps bites
Type II: Antibody Cytotoxic Hypersensitivity Reactions
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is mediated by IgM and IgG antibody reacting with
cellular or particulate antigens.
This antibody-dependent cytotoxic hypersensitivity
results in complement-mediated cytolysis,
antibody-dependent cell mediated cytotoxicity
(ADCC), or the opsonization and increased
phagocytosis of particles sensitized by antibody or
fixed complement.
Type II hypersensitivity can also damage the host
because of circulating antibodies.
Type II hypersensitivity reactions differ from type I in
three different ways:
1. Antibody combine with antigenic determinants on the
cell surface through its Fab portion before any Fc
interaction occurs.
2. Interaction of antibody with the target cell directly
result in cell death.
3. IgE is not involved
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Type III: Immune-Complex mediated
Hypersensitivity Reactions
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mediated by IgG and IgM antibodies which react
with soluble antigens.
This causes the formation of circulating immune
complexes (IC) which can cause widespread
inflammatory responses called serum sickness
Bacterial toxins and free viral proteins are presumably
the intended targets,
can cause severe organ dysfunction such as kidney
failure, rheumatoid arthritis, and types of toxic shock
syndrome.
Type IV: Cell- Mediated Hypersensitivity Reactions
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mediated by sensitized T-lymphocytes which can
cause direct target cell-mediated lysis and the release
of soluble lymphokines.
T cell arrive at a skin containing antigen after
infection producing a typical hard swelling -called
delayed reaction.
cytotoxic T-lymphocytes are important in killing cells
infected with intracellular parasites such as viruses,
bacteria, and trypanosomes as well as fungi and
perhaps cancer cells.
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T cell mediated hypersensitivity is responsible for
accelerated graft reaction and allergic reactions caused by
contact with chemical or metal allergens called- contact
sensitivity.
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