component of blood typing. It is possible to have
immune reactivity to multiple HLA proteins,
which increases the difficulty of locating a good
match for organ transplantation. Typically trans-
plant centers like to see a match on four or more
of the HLAs. A match of three or fewer strongly
suggests the recipient will reject the transplanted
organ.
See also ANTIBODY-MEDIATED IMMUNITY; ANTIGEN;
GENOTYPE; INHERITANCE PATTERN; PHENOTYPE.
humoral immunity See ANTIBODY-MEDIATED IMMU-
NITY.
hypersensitivity reaction A symptomatic inter-
action between antibodies and allergens that
causes an exaggerated and harmful response in
the body, commonly called an allergic reaction.
Hypersensitivity reactions range from mild to life
threatening in severity and symptoms.
ANAPHYLAXIS, the most severe hypersen-
sitivity reaction, is a life-threatening
emergency that requires immediate
medical attention.There are four types of hypersensitivity reac-
tion, classified according to the way in which the
ALLERGEN or ANTIGEN activates the reaction. The
classic allergic reaction is the type I hypersensitiv-
ity reaction, with exposure to an external sub-
stance (the allergen) initiating the IMMUNE
RESPONSE. Types II, III, and IV hypersensitivity
reactions are endogenous (within the body)
responsible for IMMUNE DISORDERS(other than due
to IMMUNODEFICIENCY) and AUTOIMMUNE DISORDERS.
Type I Hypersensitivity Reaction:
IgE Antibody Reaction
IMMUNOGLOBULINE (IgE), the foundation lipopro-
tein for ANTIBODY formation, mediates type I
hypersensitivity reactions. With exposure to an
external allergen, the immune response floods the
BLOOD circulation with antibodies. Mast cells,
basophils, and eosinophils (white blood cells that
have specialized immune functions) participate in
type I hypersensitivity reactions. Mast cells release
HISTAMINE, PROSTAGLANDINS, and other biochemicals
that set in motion interactions among various
proteins and cells that guide further immune
activity.
Symptoms generally occur within 15 to 30
minutes of exposure, though sometimes can
emerge 10 to 12 hours after exposure. Anaphy-
laxis (also called anaphylactic shock) is the most
severe type I hypersensitivity and is life threaten-
ing. ALLERGIC RHINITIS, ALLERGIC CONJUNCTIVITIS,
ALLERGIC ASTHMA, ATOPY, and FOOD ALLERGIES are
type I hypersensitivity reactions. Type I hypersen-
sitivity reactions tend to run in families, causing
researchers to suspect genetic underpinnings for
the allergies.
A type I hypersensitivity reaction occurs in two
stages: the induction stage, the first exposure dur-
ing which the IMMUNE SYSTEMproduces antibodies
for the particular antigen or allergen, and the elic-
itation stage, during which the immune response
activates the antibodies to attack the antigen or
allergen. There are no symptoms during the
induction stage. Each subsequent exposure to the
antigen or allergen triggers the elicitation stage,
resulting in symptoms. The elicitation stage lasts
as long as there is allergen–antibody interaction,
though symptoms may continue for some time
(hours to days) afterward.
Regardless of what form symptoms take (SKIN
RASH, tingling around the MOUTH, DIARRHEA), a type
I hypersensitivity reaction is a systemic response—
it affects and involves the body as a whole. Sensi-
tization to an allergen is long term or lifelong
because the antibody-bearing PLASMAcells (B-cell
lymphocytes that specialize to produce antibodies)
circulate indefinitely in the blood.Type II Hypersensitivity Reaction:
Cytotoxic Reaction
Immunoglobulin G (IgG) and immunoglobulin M
(IgM) mediate cytotoxic reactions, also called anti-
body-mediated hypersensitivity reactions. Type II
reactions occur as a result of interactions between
antibodies and antigens on cell membrane sur-
faces. The immune response activates the COMPLE-
MENT CASCADE, which results in the release of
biochemicals that kill the antigen-bearing cells. T-
cell lymphocytes and natural killer (NK) cells also
participate. Symptoms of a type II hypersensitivity
reaction typically emerge within a few minutes to
several hours after antibody–antigen binding.268 The Immune System and Allergies