In Canadian Constitutional law, interjurisdictional immunity is the legal doctrine that prevents a law from being applied to matters outside of the constitutional jurisdiction of the enacting head of power. For example, where a provincial law is found to intrude into a matter in the jurisdiction of the federal government the law will be interpreted through the “reading down” doctrine to exclude that matter.

Traditionally, the immunity doctrine was invoked when it was found that legislation "sterilized" or impaired the other order of government's legislative authority in a given head of power. However, the scope of the doctrine was expanded in Commission du Salaire Minimum v. Bell Telephone Co. of Canada (Bell #1) in 1966 when it was determined that a valid law could not apply if it merely affected - not impaired - a vital part of the other order of government's jurisdictional authority. In response to this more classical approach to settling matters of constitutional law, the necessary degree of infringement was revisited in Canadian Western Bank v. Alberta

Canadian Western Bank v. Alberta

Canada Western Bank v. Alberta [2007] 2 S.C.R. 3 is a landmark decision in Canadian constitutional law by the Supreme Court of Canada relating to the division of powers between Federal and Provincial legislative bodies.-Background:...

 in 2007, where the Supreme Court of Canada ruled that, in the absence of outright impairment of the "vital or essential part", Interjurisdictional Immunity would not apply.

Additionally, though the doctrine was textually justified in the Canadian Western Bank ruling, the court also expressed a preference for relying on the doctrine of Federal Paramountcy

Paramountcy

The doctrine of paramountcy is the legal principle that reconciles contradicting or conflicting laws in a federalist state. Where both the central government and the provincial or state governments have the power to create laws in relation to the same matters, the laws of one government will be...

over Interjurisdictional Immunity when attempting to resolve federalism disputes (after the impugned legislation had been found valid).

Though there remains some debate, it has generally been accepted that the doctrine applies to both the federal and provincial governments equally. Nevertheless, virtually all of the case law concerns situations where provincial laws encroach on federal matters.