A delayed or delayed allergic reaction, also called a delayed allergic hypersensitivity response , is an allergic reaction that begins between 24 and 72 hours (1-3 days) after exposure to an allergen, rather than being an immediate reaction as it occurs usual way in typical allergic reactions.
In some cases, the reaction can be delayed even more. For example, in serum sickness itcan appear up to a week later.
Delayed allergic reactions occur with many different allergens but can be difficult to diagnose, since allergic reactions are usually associated with recent exposures and with the appearance of symptoms almost immediately.
How does a typical allergic reaction occur?
An allergic reaction, or more precisely, an allergic hypersensitivity reaction , is a reaction of the immune system to a substance that is normally harmless to the body.
It must be borne in mind that not all hypersensitivity reactions are allergic, since there are hypersensitivity reactions in which the immune system does not intervene and which are not considered allergies.
In addition, allergies have traditionally been considered exaggerated reactions of the immune system , but it is important to note that it does not have to be that way.
The key to an allergic reaction is not the intensity but the response is wrong : the immune system reacts to a substance at doses generally tolerated by healthy people and harmless to them.
Substances that trigger allergic reactions, known as allergens , act as antigens. In a first contact with an allergen, for example the pollen of a plant, the antigen-presenting cells (macrophages and others) capture it, process it and present it to the helper T lymphocytes.
The T lymphocytes interact with the presented antigen and interact in turn with the B lymphocytes , which are the cells of the immune system responsible for the production of antibodies and humoral immunity .
Activated B lymphocytes are transformed into plasma cells and produce specific IgE(immunoglobulin E) antibodies against that particular allergen. Finally, this antibody is recognized by other cells of the immune system, mainly mast cells and basophils, which fix it and present it on its surface.
This process lasts a certain time (from days to years), may require several exposures to the allergen and is known as the sensitization period . During this period there are no allergy symptoms.
In a subsequent contact with the same allergen, the immune system responds directly with the specific antibody that had previously produced and is found on the surface of mast cells and basophils.
Mast cells and basophils will release histamine and other proinflammatory substances that are responsible for allergic symptoms: itchy eyes and nose, rhinitis, airway inflammation, hives, etc.
From the contact with the allergen to the appearance of symptoms, it takes a short time, between 15 and 20 minutes on average. These allergies are called immediate hypersensitivity reactions and coincide with a type I hypersensitivity reaction .
However, there are other types of allergic reactions mediated by other immunological mechanisms whose symptoms take longer to appear, from hours to days , and these reactions are known as delayed allergic reactions (not to be confused with the late phase of the reaction). type I hypersensitivity).
How does a delayed allergic reaction occur?
As mentioned, in addition to the immediate allergic reactions mediated by IgE, which are the typical allergic reactions, there are other allergic reactions that are developed by other mechanisms.
Delayed allergies are usually described as those that appear between 24 and 72 hours after exposure to the antigen, although this time is variable and may appear sooner or later. If the symptoms appear after 12 hours, it is considered a delayed reaction.
But what really defines a delayed allergic reaction is not the time in which the symptoms appear, but the mechanism by which they occur.
If immediate allergic reactions are mediated by IgE antibodies, delayed allergic reactions are mediated by other immune mechanisms that take longer to develop.
For example, in contact dermatitis, cellular immunity intervenes with the participation of CD8 + T lymphocytes, monocytes and macrophages . It corresponds to a reaction of type IV hyper sensitivity . Sensitization begins with the penetration of the allergen into the skin and its capture by antigen-presenting cells.
The antigen is presented to T cells of the local ganglia and begins to release various cytokines that produce the attraction and differentiation of CD8 + T lymphocytes that cause cell damage and the symptoms of this type of allergy. Also involved are macrophages that release hydrolytic enzymes.
CD8 + T lymphocytes, also known as cytotoxic T lymphocytes , are generally activated to fight against dangerous cells, for example fungi and bacteria, but in this case they act against foreign protein complexes formed between the allergen and skin proteins. For example, this is the mechanism of contact dermatitis for jewelery containing nickel.
Delayed allergic reactions also occur with some frequency in food allergies, in this case mediated by antibodies type IgG (hypersensitivity reaction type III) .
In other cases of delayed allergic reactions it is not known exactly what mechanism they follow, for example the allergic reactions to certain antibiotics and other medicines.