Facts on File Encyclopedia of Health and Medicine

Hemolytic ANEMIA, BLOOD TRANSFUSION reactions,
Rhesus (Rh) blood reactions (erythroblastosis
fetalis), PEMPHIGUS, GOODPASTURE’S SYNDROME, and
many DRUGallergies (notably penicillin) are type II
hypersensitivity reactions.

Type III Hypersensitivity Reaction:

Immune Complex (IC) Reaction

IgG and IgM also mediate type III hypersensitivity
reactions, though through different mechanisms
from those that occur in type II hypersensitivity
reactions. Type III hypersensitivity reactions occur
when unattached antigens enter the blood circula-
tion and activate an immune response that results
in the formation of an immune complex, a con-
glomeration of immune proteins (immunoglobu-
lins), platelets, neutrophils, and immune-related
substances that surround the antigens. Eventually
these clumps fall out of the blood circulation and
settle into tissues. Type III antibodies are autoanti-
bodies—that is, antibodies that target the body’s
own antigens.
Researchers do not know what precipitates the
immune response in most type III reactions,
though viruses such as HEPATISA, serum sickness,
and drug reactions are sometimes accountable.
Symptoms develop 3 to 10 hours after the
immune complex forms. ASPERGILLOSIS, SYSTEMIC
LUPUS ERYTHEMATOSUS (SLE), GLOMERULONEPHRITIS,
polyarteritis and other forms of VASCULITIS, and
RHEUMATOID ARTHRITISare type III hypersensitivity
reactions.

Type IV Hypersensitivity Reaction:

Delayed Reaction

T-cell lymphocytes (primarily helper T-cells) medi-

ate type IV hypersensitivity reactions, also called

delayed-type hypersensitivity (DTH) or cell-medi-

ated hypersensitivity reactions. Type IV reactions

take days to weeks to manifest. The rash of poison

ivy, poison oak, and poison sumac represents a

type IV hypersensitivity reaction. GRANULOMA is

also a typical type IV hypersensitivity reaction,

often to BACTERIAor fungi the body is unable to

completely eliminate. Common therapeutic appli-

cations of a type IV hypersensitivity reaction

include the tuberculin skin test to detect the pres-

ence of Mycobacterium tuberculosisand skin patch

ALLERGY TESTING.

Symptoms and Diagnostic Path

Symptoms vary with the type and severity of the

hypersensitivity reaction. Itching and skin rash or

URTICARIA(hives) are common with type I hyper-

sensitivity reactions. Symptoms may involve the

airways (allergic asthma) or gastrointestinal tract

(food allergies). Contact reactions typically involve

the surface of the skin though may also produce

widespread systemic symptoms. The diagnostic

path may include blood tests to assess the types

and levels of blood cells present in the circulation

as well as to detect the types and quantities of

immunoglobulins. Allergy testing can help isolate

the specific allergens for type I hypersensitivity

reactions. The doctor may conduct further diag-

hypersensitivity reaction 269

HYPERSENSITIVITY REACTION TYPES AND SYMPTOMS

Type of Reaction Symptoms Typical Onset from Exposure
type I (IgE antibodies) URTICARIA(hives), SKIN RASH, wheezing 15 to 30 minutes
itching

type II (cytotoxic) redness and swelling due to cell or tissue death minutes to several hours

type III (immune complex) redness and swelling (erythema and edema) 3 to 10 hours
pain

type IV (cell mediated) redness and hardness (erythema and induration) 48 to 72 hours (nongranuloma)
PAIN 3 to 4 weeks (granuloma)