Page "Bacillus Calmette–Guérin" Paragraph 16

# Interference by nontuberculous mycobacteria: Exposure to environmental mycobacteria ( especially M. avium, M. marinum and M. intracellulare ) results in a nonspecific immune response against mycobacteria.

Administering BCG to someone who already has a nonspecific immune response against mycobacteria does not augment the response already there.

BCG will therefore appear not to be efficacious, because that person already has a level of immunity and BCG is not adding to that immunity.

This effect is called masking, because the effect of BCG is masked by environmental mycobacteria.

Clinical evidence for this effect was found in a series of studies performed in parallel in adolescent school children in the UK and Malawi.

In this study, the UK school children had a low baseline cellular immunity to mycobacteria which was increased by BCG ; in contrast, the Malawi school children had a high baseline cellular immunity to mycobacteria and this was not significantly increased by BCG.

Whether this natural immune response is protective is not known.

An alternative explanation is suggested by mouse studies ; immunity against mycobacteria stops BCG from replicating and so stops it from producing an immune response.

This is called the blocking hypothesis.