Sally,
        you can store samples at minus 20, minus 70 or liquid nitrogen.
At minus 20 some reactions still take place and there is some loss of
materials as noted by Dennis. I suspect that there is a change in the
reactivity of some antigens but have no evidence to support this. Storage
for a few weeks should be no problem as long a you don't store them on the
bottom if this is a defrosting freezer. Would be best to avoid defrosting
freezers altogether.
Also for log term storage, it should be noted that if tissues are stored in
containers that they will become dehydrated and may dry out completely.
This dehydration is more rapid the lower the temperature. Dehydration can be
almost completely eliminated by storing samples in a container which also
has some ice cubes. (these should be free of Tequila etc.).
In general ice crystals should not form in the tissue when you go from one
temperature to another. Ice crystal damage in most cases is due to
inappropriate initial freezing of the samples or freezing thawing cycles.
Minus 20 is convenient and cheap but as noted above subject to the greatest
amount of dehydration. Minus 70 is more expensive and needs a backup power
supply and warning system if temperatures change beyond set limits.  Storage
at liquid nitrogen temperatures is less convenient but is believed to be
optimal especially for long term storage. We have some samples in liquid
nitrogen that have been stored since 1975 and are still fine (may even recut
them some day!).  However, liquid nitrogen storage is  very expensive and
requires extra care in handling.
Hope this information is useful to you
Barry