Looking for author and title of SF book with following plot.
Humans have been invaded by a Alien Galactic organization and Earth has been declared an exclusive hunting ground for alien hunters. A new alien huntsmaster has been appointed and he/it appears to be strict about putting down human resistance.
However all is not as it seems. There are different classes in the Alien Galactic Organization and the 'strongest' are repressing the 'weaker' to preserve the hierarchy. The new huntsmaster is from an reclusive alien planet that had resisted the Alien Galactic Organization for the longest period before it became part of it. The huntsmaster secretly helps humans develop their technology and transfer this knowledge to the lesser races to defeat the organization. Towards the end of the book, it is explained that humans are like white blood cells in the universal, fighting off the viruses (eg: the Alien Galactic organization).
I read this in the 90s and I was recently reminded of this story during a conversation about alien invasion themes. Thanking in advance for any clues.