Problem with < and > embedded in text

[Sinbad]

When I send to a browser an XML string like this:

ab < cd > ef

The browser outputs this:

ab < cd > ef

But when I send the browser this:

ab &lt;cd&gt; ef

instead of outputting this:

ab <cd> ef

it outputs this:

ab ef

Why? And how can I embed entities in text without their having an effect on the text?