NEW YORK (Reuters) The major active ingredient in marijuana may
reduce resistance to infection, according to laboratory studies.

Experiments with mice reveal that exposure to marijuana's
psychoactive component, THC (tetrahydrocannabinol), increased the
animals' susceptibility to the bacteria that causes Legionnaire's
disease.

Artificial THC likewise decreased the animals' immunity to
infection. The artificial form of THC is sometimes used as an
alternative to marijuana.

For 10 years, University of South Florida researchers in Tampa,
headed by Dr. Herman Friedman, chairman of medical microbiology and
immunology, have been investigating marijuana's effects on the
immune system. Recently, their governmentfunded studies have
focused on determining the molecular basis for these effects.

``We learned that THC increases the susceptibility of mice to an
opportunistic bacterial infection that a normal, healthy person
would resist,'' Friedman adds. ``This Legionella organism causes
major infections in people whose immunity (is) compromised,
including people with AIDS, older individuals, and people being
treated with chemicals to prevent organ graft (transplant)
rejection.''

In the experiments, mice infected with a dose of Legionella that
their immune systems would ordinarily defeat became susceptible to
infection when also treated with THC. The animals also developed
increased tissue levels of cytokines molecules that promote
inflammation. The mice also showed decreased Tcell activity
cells that play an important role in the immune response.

But mice treated with marijuana's nonpsychoactive cannabinoids
(cannabidiol or cannabinol) prior to being exposed to lowdoses of
Legionella did not show similar susceptibility to the infection.
And when given these marijuana components at three to four times
the concentration of THC, mice ``showed only slight or
insignificant enhancement of susceptibility to Legionella,''
Friedman says.

The researcher notes that both THC and its synthetic version
produced high levels of proinflammatory cytokines, ``but the
nonpsychoactive cannabinoids did not.''

Friedman says the findings offer new insight into how marijuana may
depress the immune system, and may have implications for marijuana
use by people whose immune systems are already compromised.