Background: The number of individuals diagnosed with allergic diseases have been steadily increasing throughout the last 60 years - with a still rising trend. The high amount of allergic cases will ultimately lead to huge socioeconomic costs for the public due to loss of productivity and decreased life quality of the affected individuals. With focus on the immunoglobulin E (IgE)-mediated reaction, which is caused by an abnormal immune response to harmless substances in the environment, specific IgE (sIgE) is developed. The interaction of sIgE with the main effector cells of allergy, mast cells and basophils, lead to the release of several biological active mediators, resulting in the development erythema, edema, pruritus and dyspnea. As the diagnosis is partially based on the detection of sIgE sensitization, which is a prerequisite for the development of an allergic reaction, it is not always clinically relevant. Thus, sIgE is not a sucient diagnostic marker, in which the need for more hazardous provocation challenges is performed. In a field with focus on personalized medicine, accuracy is important for diagnostics, however the need to discriminate between patients is also a cornerstone to obtain higher treatment effects and to streamline the resources of the health care system. The challenges described above have led to the objectives of this master thesis. Aim: This project has two main objectives. The first aim is to investigate whether a difference in mast cell reactivity and sensitivity can be established between patients with severe allergic asthma and healthy controls at different affinities of IgE using mast cell activation test (MAT), and whether this affects the release of mediators during the late phase response. The second aim is to investigate, whether MAT and basophil activation test (BAT) can be used to characterize and discriminate high from low sublingual immunotherapy (SLIT) treatment responders. Methods: To investigate mast cell response, human mast cells were isolated from CD133+ progenitor cells from either GINA 4-5 asthmatics or healthy controls. Mast cells were matured at least six weeks with cytokine-enriched media. Mast cell response with different house dust mite-specific rIgE affinity was measured by CD63+ upregulation with flow cytometry. Mast cell response with pre-AIT sera from the GT-08 study or sera and plasma from in-house birch allergic individuals was measured by sensitising mast cells over two days. Mast cells and full-blood basophils were activated with a SLIT tablet and measured by CD63+ upregulation with flow cytometry. Results: An affinity dependent difference in mast cell reactivity and sensitivity could not be established between the patient- and control group. However, an increased mean reactivity(±SE), in the mast cells sensitized with low affinity IgE, was observed for the patient group 68.84(±17.11)% com- pared to the control group 40.68(±30.77)%. Neither the baseline reactivity nor sensitivity showed any clear groupings with the global evaluation scores from the GT-08 study. Correlation was found between baseline reactivity and sensitivity for individuals suffering from birch pollen allergy, whilst no correlation was found for treatment effect. MAT and BAT indicated low comparability and poor correlation, but MAT indicated high reproducibility with <10%CD63 expression variation and a variation of 207(μDDE/ml) in sensitivity. Conclusion: Mast cell degranulation demonstrated an affinity dependent response with rIgE in sensitivity within groups, but no difference in CD63 upregulation could be established between patient- and control derived mast cells. The low affinity sensitized mast cells might indicate that asthmatics respond stronger to a reduced stimulus, but further research is needed. MAT performed with buffy coats, which have been sensitized with either serum or plasma, has proven to be reproducible. Mast cell response from pre-AIT treatment could not be established as a predictive biomarker for AIT treatment. The BAT response and its role as a predictive biomarker for AIT was inconclusive and require further research.