Type I Hypersensitivity- Immediate or anaphylactic t ype. These reactions come
on shortly after contact with an allergen and are common. Some major risks
exist in this category including anaphylaxis, bronchoconstriction and the most
deadly being anaphylactic shock (explained below). In type I hypersensitivity an
individual who has previously been sensitized to an antigen mounts a strong
immunological response in their next encounter. It is caused by an antigen
binding with a mast cell or basophil with pre-formed immunoglobulin E (IgE),
which creates a cascade response involving the release of histamine, serotonin
and other vasoactive substances. These agents also attract other white blood
cells causing further reactions. Depending on the severity of the response these
substances alter blood vessels, smooth muscle and are pro-inflammatory. These
can take the form of local or systemic responses. Local responses include, nasal
congestion and discharge, wheal and flare, and hives. Major systemic reactions
include difficulty in breathing and vascular permeability.
Many type I reactions have 2 distinct phases, the early phase and the late phase
reactions. The early phase reactions are usually noticeable within 5-30 minutes
after contact. Its effects may include vasodilation, bronchoconstriction,
increased capillary permeability, smooth muscle contraction and mucous
secretion. These may subside within 1 hour. The late phase reactions occur from
2 to 8 hours after initial exposure. These symptoms are often similar to early
phase reactions but are more intense and persistent. Late phase symptoms
include greater inflammation of tissue, sluggishness and lethargy. These
different reactions are caused by the different chemical mediators released by
the body during early and late phase contact with an antigen.
Examples of type I hypersensitivity
1) Allergic rhinitis
2) Food allergy
3) Drug allergy
4) Insect venom allergy
5) Allergic asthma
Many substances (generally proteins) can act as allergens and cause allergic
reactions. One of the frustrating aspects of treating hypersensitivity reactions is
the difficulty of figuring out the causative agent, as they are often hard to pin down.
Some common allergic reactions include;
1) Allergic rhinitis (AR) is a broad title for one of the more common allergic
reactions. Its symptoms include; sneezing, itching, nasal congestion, itchy eyes,
and rhinorrhea. Two major categories are seasonal allergic rhinitis (SAR) and
perennial allergic rhinitis (PAR). SAR is usually associated with pollen exposure
(hay fever). PAR is characterized as occurring approximately nine months a
year. Some of the allergens include; pollens, molds, animal dander, and dust
mites. AR affects an estimated 20-40 million people in the United States.