Laboratory animal allergy is a common occupational health problem in occupationally exposed individuals. Exposure is a well-defined risk factor for sensitization and asthma to rats. However the incidence of sensitization is often reduced in the highest exposed categories. One explanation is the phenomenon of high dose tolerance. It has been shown that exposure to cat allergen induces an IgG and IgG4 antibody response, without sensitization or risk of asthma. This has been explained as a modified Th2 response. We have investigated the relationship between IgE, IgG and IgG₄ and exposure to rats.

A cross sectional study was carried out on individuals exposed to laboratory animals at six pharmaceutical sites across the UK. Specific IgE was measured using RAST. Specific IgG and IgG4 antibodies to rat urinary allergen were measured in 689 and 401 exposed individuals respectively using ELISA.

Using two different measures of exposure to rats, the prevalence of sensitization increased with increasing exposure intensity. However in the highest exposure category, the prevalence of sensitization decreased. Conversely levels of IgG and IgG4 increased in parallel with increasing exposure to rats. In support of high IgG₄:IgE ratio in non sensitized individuals at highest exposure, we also observed higher IgG₄:IgE ratios in asymptomatics compared with those with symptoms.

High exposure to rats was associated with an increased production of specific IgG and IgG4 with a decreased production of IgE. The development of IgG₄ may protect against symptoms.