
What does it happen within our organism when we experience an allergic reaction? This kind of reactions can occur in different districts of our body, and they are always related to the coming into contact of an allergen (that is, a substance which is external to the body, such as pollen) with the specific antibody for that substance, produced by the organism itself.
An antigen is a substance which can cause the production of specific antibodies by the organism it puts itself in.
More precisely, an antigen which, in contact with an antibody, causes allergic emergencies is called allergen.
An antibody is a plasma protein (globulin) which is synthesized within the body by specialized cells (the plasma cells) when an external substance (an antigen) is put into it.
The antibodies which are responsible for allergic reactions are classified into two categories according to the way they appear:
- spontaneous antibodies, which are divided into natural ones, normally present in blood serum, and reaginic ones -or reagin-, which are developed by allergic subjects and circulate and fasten themselves to organs and tissues;
- induced antibodies, represented by anaphylactic ones, which appear after the injection of strong antigens, and by blocking ones, which circulate and can fasten to the antigens faster than reagin.
Immune system and allergic reactions
The tasks of immunological supervision, acknowledgment, reactivity, neutralization and elimination are performed by cells that compose the defence system of our body: T and B lymphocytes.
Under ordinary circumstances, T-lymphocytes can recognise the danger represented by an external substance. If this acknowledgment system alters, an unoffending substance can be mistaken for a harmful one:
it functions then as an allergen and triggers an immunological response that leads to the activation of cells (mastcells) and to the release of different mediators according to the allergen nature.
Some allergens typically stimulate a cell response through the activation of T-lymphocytes, which in turn produce chemical mediators, which are responsible for rash, temperature rise, swelling up and the ache around the inflammation area.
For example, a condition where such mechanism is activated is the contact dermatitis (a skin disease on an allergic basis), which shows up after the skin has come into contact with usually harmless substances, such as make-up, metals, vegetables, etc.
The allergic reactions of this kind reveal themselves after 48 hours from the contact with the antigen and this is the reason why they are referred to as delayed hypersensitivity.
On the contrary, when the allergic reaction develops within seconds or minutes from the contact with the antigen, we are in presence of immediate hypersensitivity, which is due to the presence of a specific range of immunoglobulins, the lgE ones.
LgE antibodies are produced even under ordinary conditions, but only in a small quantity. In allergic subjects, instead, their concentration is usually very high. Once produced -as a consequence of an antigenic impulse-, the lgE antibodies fix themselves to the tissue of other cells, the mastocites, located in organs rich in connective tissue, which contain substances capable of quickly inducing an inflammatory reaction, such as the histamine.
When the allergen combines with the lgEs fixed to the tissue, the mastocite activates, releasing the histamine (degranulation), which in turn triggers the inflammation (urticaria, asthma, rhinitis).
At the same time, other inflammatory cells are activated and “hired” in the area of the allergic reaction, with the release of other mediators.
Related article: The allergies articole 2.