IN THE MEXICAN crisis to far, at least -- the world's financial system has been working as it is supposed to. The safeguards are holding. For Mexicans, the real tests lie ahead as the new government begins its struggle to close an enormous deficit. For everybody else, there is the sharpening realization that Mexico is not an isolated case.

The Reagan administration has moved rapidly and effectively to help Mexico. First, it extended enough dollars, through loans and accelerated oil purchases, to stave off a panic. Now, the Federal Reserve Board is publicly encouraging the commercial banks not to cut off lending. Meanwhile, the International Monetary Fund is preparing a larger loan for the longer haul. That will get Mexico past the immediate peril. But there is likely to be a series of other, similar crises ahead.

International borrowers and lenders have fallen into a trap. Banks in rich countries lent heavily to poor countries, which intended to use much of the money to build up their exports and make themselves less poor. But as those huge debts mounted, the rich countries began a long struggle to get inflation under control. Interest rose, and a worldwide recession began. For the poor countries, the burden of interest payments shot upward while the markets for their exports fell. That now jeopardizes banks in the rich countries, raising the possibility of a credit collapse -- a disaster for more than the bankers.

The way out of the trap is to keep moving on the present route. On Friday, the Federal Reserve brought its discount rate down another half point. The IMF is doing its job, lending where it's needed most. When a recovery starts here and in Europe, the Third World countries' export earnings will pick up again -- unless, in the meantime, American and European protectionists have succeeded in locking the door against them. That's one reason why it's desperately important to keep the protectionists from succeeding in the bad months of high unemployment immediately ahead. With steady nerves and a little luck, governments can keep lines of credit open to support trade as it revives -- and that's the only real hope for bringing unemployment down.

The thought for Americans to keep firmly in mind is that while Latin American economies depend on the United States, the reverse is also true. A collapse of credit there would be very bad for employment here. Neither the First World nor the Third World can find its way out of the trap without the other.