An allergy is a reaction of the body's immune system to commonly harmless substances in the environment called allergens. Allergy is characterized as a type I hypersensitivity reaction in which specific blood cells, basophils and mast cells, play a leading role, as well as a specific type of antibody, immunoglobulin E. Foods (eg fish, eggs), drugs (eg fish) insect bites, pollen, paints, dust, animal hair, contrast media used in imaging tests, cosmetics, etc.

Allergens (antigens) reach the body through respiration, skin contact or even through food or injection. When the body meets a foreign substance that acts as an allergen, its immune system activates the defense mechanisms of humoral and cellular immunity to neutralize the antigen. In response to allergens, the body's immune system produces antibodies, which have a specific defense depending on the antigen.

Symptoms depend on the substance involved and can affect the airways, skin or even the digestive system. In some cases, allergies can trigger a life-threatening reaction, also known as anaphylaxis. They are usually manifested by itching , sneezing, runny nose, itching in the nose, eyes or upper mouth, erythema (redness), irritation (stinging) or tearing or swollen eyes. In the case of skin allergies, such as atopic dermatitis, itching, redness and peeling of the skin are observed.

Allergic rhinitis (AR) is an atopic disease characterized by symptoms of nasal congestion, clear rhinorrhea, sneezing, postnasal drip, and nasal pruritis.

In addition to nasal symptoms, patients with Allergic Rhinitis may also experience associated allergic conjunctivitis, nonproductive cough, eustachian tube dysfunction, and chronic sinusitis. AR can be classified as either seasonal (intermittent) or perennial (chronic).

Διαλείπουσα Αλλεργική ρινίτιδα ονομάζεται η ρινίτιδα κατά την οποία η συμπτωματολογία εμφανίζεται με περιοδική ή εποχική κατανομή και οφείλεται κυρίως σε αλλεργιογόνα γύρεων αλλά μπορεί και σε μύκητες. Τα συμπτώματα είναι  καταρροή, πταρμοί, Ρινική Συμφόρηση, Ρινικός και Οφθαλμικός Κνησμός, Δακρύρροια, Επιπεφυκίτιδα

Συνεχής ή Ολοετής (Χρόνια) Αλλεργική Ρινίτιδα είναι αλλεργική ρινίτιδα κατά την οποία η συμπτωματολογία εμφανίζεται με σχεδόν ολοετή κατανομή. Τα συμπτώματα είναι παρόμοια με την εποχική ρινίτιδα αλλά κυριαρχεί η  ρινική συμφόρηση και συνήθως απουσιάζουν τα οφθαλμικά συμπτώματα. Οφείλεται κυρίως σε ενδοοικιακά  αλλεργιογόνα, τα οποία προέρχονται από τα ακάρεα της οικιακής σκόνης, τα κατοικίδια ζώα και την κατσαρίδα. Ο ασθενής εμφανίζει χρόνια ρινική συμφόρηση, πταρμούς, ρινόρροια, ρινικό και οφθαλμικό Κνησμό, καθώς και δακρύρροια.

Irritated skin can be caused by a variety of factors. These include immune system disorders, medications and infections. When an allergen is responsible for triggering an immune system response, then it is an allergic skin condition.

Skin allergies include diseases such as urticaria, atopic dermatitis, angioedema and allergic contact dermatitis. Virtually any allergy (with possible exception that of respiratory allergy) can occur either exclusively as a skin allergy or with a combination of symptoms from the skin and other organs.

In fact, if the skin allergy (urticaria) is combined with simultaneous symptoms from other organs, then the condition is called systemic anaphylactic reaction (anaphylaxis).

Urticaria is a disease characterized by the development of wheals (hives), angioedema, or both. Urticaria needs to be differentiated from other medical conditions where wheals, angioedema, or both can occur as a symptom, for example skin prick test, anaphylaxis, auto-inflammatory syndromes, or hereditary angioedema (bradykinin-mediated angioedema).

A wheal consists of three typical features:
1. It is characterized by a central swelling of variable size, almost invariably surrounded by a reflex erythema.
2. It is associated with itching or sometimes a burning sensation.
3. It has a fleeting nature, with the skin returning to its normal appearance, usually within 1 - 24 h. Sometimes wheals resolve even more quickly.

Urticaria is distinguished into acute urticaria, when it lasts less than 6 weeks and in chronic urticaria, when there is daily or almost daily appearance of new wheals, for a period of more than 6 weeks.

Angioedema is defined as "subcutaneous tissues and/or submucosal tissues circumscribed non-pitting edema affecting lips, face, neck, and extremities oral cavity, larynx, and gut." It becomes life-threatening when it involves the larynx, while intestinal angioedema is painful and mimics acute abdomen.