Allergen extracts are complex mixtures of proteins and contain varying amounts of allergenic and non-allergenic components. Many factors such as the biovariability, differences in extraction process and subsequent handling of allergens can affect the final composition, potency, and stability of allergen preparations. Genetic diversity of affected people adds another level of complexity. In order to control variability and to achieve consistency and reproducibility for optimal safety and sensitivity/specificity, it is essential to standardize the amount of allergen used in prick tests. Therefore, the system for biological standardization mainly used in Europe still is the biological calibration of in-House Reference Preparations (IHRP). The method has been adopted by the Nordic Council on Medicines as the Nordic Biological Unit, Histamine Equivalent Potency (HEP) or Skin Prick Test (SPT) value. The aim of this procedure is to estimate the biological activity of allergen extracts. The activity of an allergen extract is defined as 1 SPT per ml, when the extract provokes a specific skin reaction with a wheal of the same size as a wheal provoked by reference histamine at a concentration of 10 mg/ml, when both solutions are administrated using the same technique (prick testing) on at least 20 individuals who are sensitized to the allergen concerned.

The present study aims to standardize the allergen extracts of Dactylis glomerata, Lolium perenne, Secale cereale y Olea europaea by using this method.

Inclusion Criteria:

A. Subjects sensitized to one or more allergen extracts:

1. Positive clinical history with inhalant allergy to at least one of the allergen to be standardized.
2. At least one positive prick test (mean wheal diameter greater or equal than 3mm) .
3. Positive prick test (wheal diameter greater or equal to 3 mm) to Histamine 10 mg/ml
4. Age: 18-60 years
5. Written informed consent
6. Patients will preferably be monosensitized, or with clinically relevant sensitization , or with primary sensitization (maximum cutaneous reactivity) to the extracts under investigation.
7. Women of child-bearing potential must have a negative urine pregnancy test at the time they begin the study.

B. Atopic Subjects:

1. Age: 18-60 years
2. Written informed consent
3. Positive prick test (wheal diameter greater or equal to 3 mm) to Histamine 10 mg/ml
4. Negative prick test (mean wheal diameter less or equal to 3mm) when tested with already standardized extracts of the allergens under investigation or with cross reactive extracts.
5. Women of child-bearing potential must have a negative urine pregnancy test at the time they begin the study.

C. No Atopic Subjects:

1. Age: 18-60 years.
2. Written informed consent
3. Positive prick test (wheal diameter greater or equal to 3 mm) to Histamine 10 mg/ml
4. Negative prick test (mean wheal diameter less or equal to 3mm)to all allergens tested in diagnostic prick test.
5. Women of child-bearing potential must have a negative urine pregnancy test at the time they begin the study.

Exclusion Criteria (shared by the 3 groups):

1. Immunotherapy in the 2 years prior to the study against either the allergen to be tested or an allergen which is cross-reactive. (it is not applicable to no atopic patients)
2. Any drug which may interfere with the cutaneous test or with its result.
3. Any medical condition that from investigator's point of view the skin prick test cannot be done .
4. Women who are pregnant or breast-feeding or are child-bearing age and who have not demonstrated that they have been surgically sterilized or have other means of not bearing children.

4.Subjects who have participated in another clinical trial within 3 months prior to this study.