<a href="#" onClick="getData('revision.php?mid=3&sesion=2&user=1', 'messageContent');">

var XMLHttpRequestObject = false;

try {
	XMLHttpRequestObject = new ActiveXObject("MSXML2.XMLHTTP");
} catch(exception1) {
	try {
		XMLHttpRequestObject = new ActiveXObject("Microsoft.XMLHTTP");
	} catch(exception2) {
		XMLHttpRequestObject = false;
	}
}

if (!XMLHttpRequestObject && window.XMLHttpRequest) {
	XMLHttpRequestObject = new XMLHttpRequest();
}

function getData(dataSource, divID) 
{
	if(XMLHttpRequestObject) {
		var obj = document.getElementById(divID);
		XMLHttpRequestObject.open("GET", dataSource);
		
		XMLHttpRequestObject.onreadystatechange = function()
		{
			if (XMLHttpRequestObject.readyState == 4 && XMLHttpRequestObject.status == 200) {
				obj.innerHTML = XMLHttpRequestObject.responseText;
			}
		}
		
		XMLHttpRequestObject.send(null);
	}
}