The challenges faced by our immune system every day is not easy to comprehend.
The immune system battles germs, particles and toxins unbeknownst to us and only
alerts us when it is overwhelmed, at which point we resort to our trusted
Hives antibiotics and other drugs. The fact that the human body has the ability to
initiate and mount an immune response against many diverse types of microbes in
the environment is one of the factors that make us, humans, one of the great marvels
of this world. Yet even a system as efficient as our immune system is prone to errors.
At certain times, our body perceives innocuous particles and substances as harmful
and produces immune effectors against these. In such cases, the foreign particle itself
does little harm, but the body ends up harming itself instead. Such a reaction is known as allergy
Allergy-An Immune Hypersensitivity
Allergy, also known as hypersensitivity is a common term used to describe a set of reactions
that are produced in the body in response to an allergen, which can range from mild to
life-threatening conditions. When an allergen enters the body, they trigger a cascade of immune
reactions that includes the production of the antibody IgE which stimulates other cells to release
their own chemicals, further instigating the hypersensitivity reaction. It is interesting to note that
the response occurs in those areas through which a pathogen may enter our body hence the sneezes,
the eye watering and skin reddening, all are attempts of the body to rid of the intruder, itself.
Other parts where the symptoms occur are the sinuses, ears, and the stomach lining. In some cases,
it may trigger or worsen the symptoms of asthma. It is not clear what exactly activates an allergic
response against a particular substance. In some cases, allergies can as well be hereditary.
Allergic Response- The Workings
There are four kinds of allergic (hypersensitivity) reactions:
Type 1 (Anaphylactic reaction)
Antibodies are proteins that bind to foreign substances (antigens) and mediate their removal
from the body. Among the five classes of antibodies, majority of the allergic responses are
mediated by IgE (Immunoglobulin E). On the first exposure to the allergen, these IgE antibodies
get sensitized to the allergen. In the event when body is exposed to the same allergen for a
second time, IgE antibodies recognize them and mount an immune response against them.
IgE antibodies present on the surface of immune cells are cross-linked when an allergen binds
to them. This cross-linking activates the immune cell, the first of which are the mast cells. After
activation, mast cells release the chemicals stored in small intracellular granules, namely, leukotrienes,
histamines, serotonin, bradykinin and cytokines. These mediators act on nearby cells, creating the
signature symptoms associated with allergies, including reddening of skin, inflammation and pain.
Cytokines are chemical messengers which further activate other nearby cells and inflate the reaction
sometimes to systemic levels. The other immune cells that are further activated in allergic response
are basophils and eosinophils, both of which also have intracellular granules with chemical mediators
of immune reaction. Normal response of these cells is usually against parasites and other infectious
agents. The severity of the allergic response in case of IgE mediated mast cell activation depends upon
the amount of antigen and its route of entry.
Type 2 (Cytotoxic reaction)
Cytotoxic reaction is mediated by IgG and IgM antibodies, which binds to antigens on the surface
of healthy cells and result in their lysis (breakdown).
Type 3 (Immune complex reaction)
IgG and IgM antibodies bind with the antigen (the allergen) and form immune complexes which
get deposited at different sites in our body. The presence of these complexes triggers an immune
response at those sites which leads to damage in the healthy tissues of these sites. This is seen
in case of some autoimmune diseases like lupus.
Type 4 (Delayed type hypersensitivity)
This type of allergic reaction is not carried out by antibodies, but instead by cells of immune
system including T cells and macrophages. It is called delayed type hypersensitivity because
its effects are observed 48 to 72 hours after exposure. An example of type 4 hypersensitivity
reaction is the Tuberculin Skin Test (TST) for determining the exposure to Tuberculosis.
A positive test is characterized by the formation of red bump (wheal) on the skin where
the protein was injected
Common Allergic reactions
Some of the major types of allergic reactions can be distinguished based
on the source of the allergy and the symptoms they produce.
Seasonal allergic rhinitis, also commonly known as hay fever, is characterized by sneezing,
runny nose and itching of the nose and eyes. It occurs mostly during pollination season
where the presence of pollen in the air triggers these reactions. If these symptoms occur
all year round, they are likely caused due to an allergen that may be present indoors such
as dust, fungal spores, or pets.
Hives are a type of allergic reaction symptomized by itching and reddening of skin and the
formation of small bumps on the skin, often resulting from a food or medication allergy.
A similar condition is contact dermatitis, which presents a rash when a substance
(found in soaps, cosmetics, jewelry or perfumes) comes in contact with skin.
Eye allergy or allergic conjunctivitis, is the reddening and watering of the eyes in response to an allergen.
Drug allergies, which are different from drug side effects, may cause hives, rash or fever.
Certain medications are more likely than other to cause drug
allergies- antibiotics, pain killers, chemotherapeutic drugs, etc.
Anaphylaxis is a life-threatening condition that arises due to exposure to an allergen.
It results in throat constriction, causing breathing difficulties. Other symptoms include
nausea, abdominal cramps, vomiting, rapid pulse, drop in blood pressure and seizures.
Anaphylaxis triggers are food, insect sting, medication and latex.
It requires immediate medical attention.
Testing for Allergies-An Option
There are two ways to test for allergies. The first is the skin test and
the second method is blood testing.
- Skin Test
A minute amount of the suspected allergen is injected into
the skin and the region is observed for the development of swelling or redness.
Blood Testing Commonly includes the following tests:
- IgE Antibody
Total IgE test measures the amount of IgE antibody in the bloodstream; a higher than normal
level indicates a recent allergic reaction. Specific IgE test can help identify the allergen that triggered the reaction.
- Histamine
Normally, the concentration of histamines in the blood is very low, its levels elevate in
case of anaphylactic reactions and hence, can be used to test for it.
- White Blood Cell Count (WBC Count)
Among the WBC, eosinophils and basophils are mediators of
immune reaction; elevated counts of these cells in the
blood can point towards a recent allergic response.
Blood tests for allergies are done by Radioallergosorbent
Testing (RAST), ELISA and ImmunoCAP testing
techniques. These tests have the advantage over skin test
as there is no chance of severe reaction due to the
iniection of allergen into the skin.
An Ounce of Prevention
The only way to prevent an allergic response is to avoid the allergen.
In many instances, this may involve even staying away from beloved pets or food that may trigger
this response. In cases, where exposure is unavoidable, such as when the allergy is due to dust,
certain medications such as anti-histamines are recommended to manage the symptoms.
Prior knowledge of drug allergies is very important in case of people undergoing treatment
or surgeries. Severe allergic reaction that lead to anaphylaxis, are medically treated with
epinephrine.
More important than any of these is being aware and educating loved ones
regarding your allergies, and informing any prior incident of severe allergic reaction.
Immediate medical attention is required for severe allergic reactions and thus, recognizing
its symptoms can ensure that treatment is received in time.
