Rituximab is a relatively new medicine which works on the immune system.
https://www.gosh.nhs.uk/medical-information/medicines-information/rituximab-chronic-inflammatory-conditions
Is "which works..." a non-defining relative clause modifying the noun "medicine"?
Would "that" be equally correct in place of "which" here?
No: it's a defining (or restrictive) relative clause -- note the absence of a comma after "medicine". It modifies the nominal "relatively new medicine". Yes, you could replace "which" with "that".
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No: it's a defining (or restrictive) relative clause -- note the absence of a comma after "medicine".
It modifies the nominal "relatively new medicine".
Yes, you could replace "which" with "that".
(Note that the terms 'defining' and 'restrictive' have the same meaning)