Well although i dont use the Ajax command the following seems to imply that you can load a variable with the contents of the LastData expression. Also if the LastData expression is more complex i.e consists of multiple pieces of data then you can use the tokenat system expression to break it into smaller pieces.
The basic usage of the AJAX object consists of:
Use the Request action to load a URL.
A moment later after the request completes, On completed triggers.
The LastData expression can be used to access the content of the response.
The tokenat system expression may be useful to split simple responses. Alternatively, you could load LastData in to the XML plugin to read it as an XML document.