Exactly what biochemical factors control the intensity of an allergic reaction?
Network of possible factors that contribute to the variability of the immune reaction between individuals is still not well investigated.

Type I hypersensitivity is mediated by IgE antibodies, which means that lymphocytes B after initial contact with antigen undergo oligoclonal selection. Only these with the highest affinity towards the antigen will survive. During this process (see: germinal centre reaction) genomic regions responsible for antigen binding change their sequence in non-deterministic manner. So, each individual has slightly different set of antibodies targeting the same antigen.

Moreover, within the same antigen (allergen), different regions (epitopes) can trigger the immune reaction in different individuals. 

Other, more general factors also contribute to the symptoms associated with hypersensitivity reaction. For example: hormones, infections, medications, microflora condition, genetics (eg. major histocompatibility complex) or even psychological status. 

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