The effect of increasing sibling number was strongly associated with reduced ryegrass specific sensitisation (p=0.001) but not HDM-specific sensitisation (p=0.80). The difference in the sibling effect for HDM-specific and ryegrass-specific sensitisation was significant (p=0.02).

The observation that larger family size had a clear protective effect for ryegrass-specific sensitisation but not HDM-specific sensitisation, if confirmed in other work, suggests that the apparent protective effect of high sibling number does not occur to the same extent for all allergens.

The main strength of this study was the delineation of mutually exclusive sensitisation outcomes, allowing a clearer assessment of the effect of family size on allergen-specific sensitisation without any possible contamination due to concomitant sensitisation to other allergens.