TYPE IV HYPERSENSITIVITY
Type IV hypersensitivity is also known as cell
mediated or delayed type hypersensitivity. The classical example of this
hypersensitivity is tuberculin (Montoux)
reaction which peaks 48 hours after the injection of antigen (PPD or old
tuberculin). The lesion is characterized by induration and erythema.
Table
3 - Delayed hypersensitivity reactions
|
Type
|
Reaction
time
|
Clinical
appearance
|
Histology
|
Antigen
and site
|
contact
|
48-72 hr
|
eczema
|
lymphocytes, followed by
macrophages; edema of epidermis
|
epidermal ( organic
chemicals, poison ivy, heavy metals, etc.)
|
tuberculin
|
48-72 hr
|
local induration
|
lymphocytes, monocytes,
macrophages
|
intradermal (tuberculin,
lepromin, etc.)
|
granuloma
|
21-28 days
|
hardening
|
macrophages, epitheloid
and giant cells, fibrosis
|
persistent antigen or
foreign body presence (tuberculosis, leprosy, etc.)
|
Type IV hypersensitivity is involved in the
pathogenesis of many autoimmune and infectious diseases (tuberculosis, leprosy,
blastomycosis, histoplasmosis, toxoplasmosis, leishmaniasis, etc.)
and granulomas due
to infections and foreign antigens. Another form of delayed hypersensitivity is
contact dermatitis (poison ivy, chemicals, heavy metals, etc.)
in which the lesions are more papular.
Type IV hypersensitivity can be classified into three categories depending on
the time of onset and clinical and histological presentation.