Title

something like /ddd setResistance <target> <dddtype (has auto complete)> <amount>

and /ddd setImmunity <target> <dddtype> <true|false>

Better syntax:
/ddd resistance <set|add> <target> <dddtype> <amount>
/ddd immunity <give|take|toggle> <target> <dddtype>

Might not be needed, but some /ddd query <immunity|resistance> where if resistance:

/ddd query resistance <target> <dddtype> <amount> <eq|gt|lt|ge|le>

and if immunity:

/ddd query immunity <target> <dddtype> <true|false>

/ddd query would behave similar to /testfor