Table with Checkboxes - selected all Checkboxes and pass values to AJAX script

I have a table with a checkbox in the first column - when checked this performs an AJAX script that updates a PHP session variable with the selected values. This is all working well but I now need to extend this to have a check box at the top of the first column to allow the user to select all items in the table and pass the value of the selected items (e.g. comma separated) as a parameter to the AJAX script - assuming I need a new script just for this.

Here's what I have so far:

$(document).ready(function() {
  $("input.select-item").click(function() {
    var productID = $(this).val();
    // Create a reference to $(this) here:
    $this = $(this);
    $.post('productSelections.php', {
      type: 'updateSelections',
      productID: productID,
      selectionType: 'single'
    }, function(data) {
      data = JSON.parse(data);
      if (data.error) {
        var ajaxError = (data.text);
        var errorAlert = 'There was an error updating the Product Selections';
        $this.closest('td').addClass("has-error");
        $("#updateSelectionsErrorMessage").html(errorAlert);
        $("#updateSelectionsError").show();
        return; // stop executing this function any further
      } else {
        $this.closest('td').addClass("success")
        $this.closest('td').removeClass("danger");
      }
    }).fail(function(xhr) {
      var httpStatus = (xhr.status);
      var ajaxError = 'There was an error updating the Product Selections';
      $this.closest('td').addClass("danger");
      $("#updateSelectionsErrorMessage").html(ajaxError);
      $("#updateSelectionsError").show();
    });
  });
});

<table class="table table-condensed table-striped table-bordered">
  <thead>
    <th><input type="checkbox" class="select-all checkbox" name="select-all" /></th>
    <th class="text-center" scope="col">Product ID</th>
    <th class="text-center" scope="col">Description</th>
  </thead>
  <tbody>

    <tr class="" id="85799">
      <td id="AT36288"><input type="checkbox" class="select-item checkbox" name="select-item" value="AT36288" /></td>
      <td>AT36288</td>
      <td>Apples</td>
      <td></td>
    </tr>
    <tr class="" id="85800">
      <td id="AT36289"><input type="checkbox" class="select-item checkbox" name="select-item" value="AT36289" /></td>
      <td>AT36289</td>
      <td>Bananas</td>
      <td></td>
    </tr>
    <tr class="" id="85801">
      <td id="AT36290"><input type="checkbox" class="select-item checkbox" name="select-item" value="AT36290" /></td>
      <td>AT36290</td>
      <td>Oranges</td>
      <td></td>
    </tr>
    <tr class="" id="85803">
      <td id="AT36292"><input type="checkbox" class="select-item checkbox" name="select-item" value="AT36292" /></td>
      <td>AT36292</td>
      <td>Grapes</td>
      <td></td>
    </tr>
  </tbody>
</table>

I've added a checkbox in the first row that can be used to select all items - essentially the equivalent of clicking each checkbox one by one. Not sure how to either extend or create a new script that when clicked checks each checkbox and passes the ID values to a variable that I can include in my AJAX script?