Even in the absence of express treaty comity between countries foreign to each other usually involves during time of peace the privilege of the subjects of one country to come into the territory of the other, to conduct business and acquire property, and to have the protection of the judicial tribunals to substantially the same extent as enjoyed by subjects of the latter; especially is this true where the subjects of one country become permanent residents of the other, although they may not acquire citizenship. The only substantial exception to this rule of comity is as to the ownership of real estate. By the law of England, as it existed when the colonies became independent, an alien could not, in general, acquire title to real estate by inheritance. In many of the states the disability of aliens to acquire and hold real estate has been removed, while in others it has been preserved.

In order that such questions as this might not be left uncertain as between the states, and dependent on comity merely, it was provided in the federal constitution that "The citizens of each state shall be entitled to all privileges and immunities of citizens in the several states" (Const. Art. IV, § 2, ¶ 1). The manifest result is that all the privileges and immunities enjoyed by the citizens of a state by virtue of the fact of citizenship may also be enjoyed by citizens of any other state; and this involves a prohibition of any discrimination in the laws of a state against citizens of other states as compared with the state's own citizens. Thus a state statute is unconstitutional which restricts the pursuit of some particular business to citizens of the state and prohibits citizens of other states from engaging in such business.

But political privileges, such as the right to vote and hold office and the privilege of serving on juries and the like, can be limited to citizens of the state, and a person going from one state to another does not carry into the latter state the privileges which he enjoyed in the former, for these are matters to be regulated by each state for itself. The privilege of practising law in a state may be limited, undoubtedly, to persons who are citizens of the state. There are some privileges also, such as the right to fish in the public waters and to hunt game within the state limits (McCready v. Virginia), which are regarded as incident to the enjoyment by the citizens or inhabitants of the state of the public property belonging to the state as representing its people, and the state may exclude from the enjoyment of these privileges persons who are not inhabitants of the state. The following are privileges and immunities of citizenship which cannot be denied by a state to citizens of other states: protection by the government; enjoyment of life and liberty; the right to acquire and possess property, subject, of course, to the general police power, which, however, must be exercised without discrimination as between citizens of the state and citizens of other states; the right of a citizen of one state to pass through or reside in any other state; the right to institute and maintain actions in the courts of the state ( Ward v. Maryland).

The equality of privileges thus guaranteed is equality with citizens, that is, with natural persons who are entitled to citizenship in the state in which they have their permanent residence. Corporations are artificial persons, and while they are regarded as having in many respects the same rights as natural persons and protected by the guaranties of rights which are accorded to natural persons, they are not and cannot be citizens in the full sense and meaning of the term. Therefore, a state is not bound by the provisions of the federal constitution to accord to corporations created in other states all the privileges granted to state corporations, much less all the privileges and immunities possessed by natural citizens. While it is usual for a state to allow corporations created in other states to carry on business in that state, it is not a violation of the provision of the federal constitution to impose restrictions on such foreign corporations which are not imposed on corporations created in the state, or to discriminate as against foreign corporations in favor of domestic corporations; and it is regarded as permissible to a state to exclude foreign corporations altogether or to prescribe special conditions on which all foreign corporations or foreign corporations doing a particular class of business shall be allowed to engage in business within the state (Paul v. Virginia and Blake v. McClung). It does not follow that a foreign corporation can be deprived of property lawfully acquired within the state without due process of law, or that it can be denied within the state the equal protection of the laws as to property which it has lawfully acquired.