Interfacing an application running on a Windows server to an application running on a Linux server

We are looking into writing a basic service on a Windows server that takes a text string and sends it to a service running on a Linux server.  The Linux server then needs to call another local application, take the response from that application and send a text string back to the Windows server.  

I've not done this before (although I'm fairly comfortable with C#), but could really do with a good starting point on the Linux side of things.  For example, are there any services built into Linux that would facilitate this (or third party products etc)?

In short - I just need a bit of guidance to start off with.,=