Purpose of review: The 'hygiene hypothesis' suggests that a relationship exists between improved hygiene and an increase in allergic diseases. As an underlying mechanism for this hypothesis it is proposed that due to the lack of microbial stimulation either a misbalance in T helper type responses or a misbalance in regulatory mechanisms develops. As yet, however, a specific infectious factor responsible for the hygiene hypothesis has not been found.
Recent findings: Animal models have lent support for mycobacteria as important candidates in the hygiene hypothesis. These animal studies have also suggested that mycobacterial treatment generated regulatory mechanisms which restored the immune balance. In contrast, the relationship between mycobacterial infection or treatment and the development of allergy and asthma in humans is unclear and highly controversial.
Summary: Mycobacteria have been found to unambiguously reduce allergic and asthmatic manifestations, suggesting that mycobacteria perhaps can be used as an 'anti-asthma' vaccine. Conflicting results in humans, however, confirm that the complex and multifactorial interactions between the environment and the genetic background of the individual contribute to the development of allergic disease. Therefore, the hygiene hypothesis should involve the genetic and the environmental background of the individual.