Type I Hypersensitivity Flashcards
The type I hypersensitivity reactions are commonly thought of as_______
allergies
Some examples of type I hypersensitivity reactions
hay fever,
allergic asthma,
hives, and
systemic anaphylaxis
antigens that trigger type I hypersensitivity
allergens
Examples of common allergens include
peanuts, eggs, and pollen
short time lag, usually minutes, between exposure to allergen and the onset of clinical
symptoms
distinguishing feature of type I hypersensitivity
The first clue about the cause of type I hypersensitivity was provided by
Carl Wilhelm Prausnitz and Heinz Küstner
It occurs when serum is transferred from an allergic individual to a non-allergic individual, and the second individual is challenged at a later time with the specific allergen.
passive cutaneous anaphylaxis
*least abundant antibody class in the serum, normally accounting for less than 1% of all the immunoglobulins
*not involved in typical immune responses such as complement fixation and opsonization
*unique in its ability to bind to specific receptors on mast cells and basophils
*play a major role in type I hypersensitivity allergic reactions and in defense against parasites
IgE
Greek word atopos (meaning “out of place”), refers to an inherited tendency to develop classic allergic responses to naturally occurring inhaled or ingested allergens
Atopy
The key immunologic components involved in type I hypersensitivity reactions are
IgE antibody,
mast cells,
basophils, and
eosinophils
The response begins when a susceptible individual is exposed to an allergen and produces specific IgE antibody. IgE is primarily synthesized by
B cells and plasma cells in the lymphoid tissue of the respiratory and gastrointestinal tracts, as
well as the lymph nodes
The regulation of IgE production appears to be a function of a subset of
T cells called type 2 helper cells (Th2)
This Th2 type of response results in production of several cytokines, including
IL-4 and IL-13
the IgE antibody attaches to high-affinity receptors called FcεRI, which bind the fragment crystallizable (Fc) region of the epsilon-heavy chain
sensitization phase
Large numbers of these receptors are found on mast cells and basophils, with a single cell having as many as ______ such receptors
200,000
cells internalize and process allergens from the environment and transport the allergen-MHC class II complex to local lymphoid tissue where synthesis of IgE occurs
Langerhans and dendritic
Binding of IgE to cell membranes increases the half-life of the antibody from
2 or 3 days to at least 10 days
*principal effector cells of immediate hypersensitivity
*These cells are found throughout the body and in most organs tend to be concentrated around the small blood vessels, the lymphatics, the nerves, and the glandular tissue
*have abundant cytoplasmic granules that store numerous preformed inflammatory mediators
*present in the peripheral blood, where they represent less than 1% of the total white blood cells (WBCs)
Mast cells
respond to chemotactic stimuli during inflammation and accumulate in the tissues, where they can persist for a few days
Basophils
*adjacent cell-bound IgE molecules are cross-linked by a bivalent or multivalent antigen, causing aggregation of the surface FcεRI receptors
*initiates complex intracellular signaling events involving multiple phosphorylation reactions, an influx of calcium, and secretion of cytokines
activation phase
*most abundant preformed mediator
*comprises approximately 10% of the total
weight of the granules in mast cells
histamine
These preformed substances are referred to as
primary mediators
Other primary mediators include
heparin,
eosinophil chemotactic factor of anaphylaxis (ECF-A),
neutrophil chemotactic factor, and
proteases
Release of these substances is responsible for the early-phase symptoms seen in allergic reactions, which occur within ____ ____ minutes
after exposure to the allergen
30 to 60
mediators bind to receptors on target organs, most notably the skin, respiratory tract, and gastrointestinal tract, producing symptoms characteristic of an allergic response
chemical
The clinical manifestations depend on the target tissue and type of receptors activated. For example, in the skin, local swelling and redness, sometimes referred to as a
wheal-and-flare reaction
Contraction of the smooth muscle in the bronchioles may result in
airflow obstruction
can cause hypotension or shock
Increased vascular permeability
*In addition to immediate release of preformed mediators, mast cells and basophils are triggered to synthesize other reactants from the breakdown of phospholipids in the cell membrane.
*In this phase of the reaction, numerous cells, including eosinophils, neutrophils, Th2 cells, mast cells, basophils, and macrophages, exit the circulation and infiltrate the allergenfilled tissue
late-phase
newly formed, or secondary, mediators include
plateletactivating factor (PAF);
prostaglandin (PG) D2;
leukotrienes (LT) B4, C4, D4, and E4; and cytokines
secondary, mediators are more potent than the primary mediators and are responsible for a late-phase allergic reaction that can be seen in some individuals __ __ hours after exposure to the antigen
6 to 8
play an important role in the late-phase reaction. These cells normally compose 1% to 3% of the circulating WBCs
Eosinophils
During allergic reactions, IL-5 and other cytokines released from the Th2 cells stimulate the bone marrow to increase production of eosinophils, and the number in the peripheral blood increases, producing
eosinophilia
In individuals with persistent inflammation resulting from the late-phase reaction, such as those with
chronic asthma, tissue remodeling can result
