The only studies claiming this effect on animals used near- leathal doses. Two studies, one in 1978 and another in 1988 have shown Marijuana to stimulate the immune system. This is true but misleading. Any imparment of the short term memory dissappears when the one is no longer under the infuence of marijuana. Often, the short-term memory effect is paired with a reference to Dr. Heath's poor rhesus monkeys to imply that the effect is permanent.
History documents the opposite effect.
The dutch partially legalized marijuana in the 1970's. Since then, use of hard drugs has declined substantially. Marijuana tends to substitute for much more dangerous drugs. There is no such thing as a harmless drug, but marijuana is is far less harmful than either alcohol of tobacco. I belived rather naively that once poeple understood that it would be legalized within ten years.