And because there are two different types of T cell receptors we categorize T lymphocytes into two categories. We have AlphaBeta T cells which consist of special T cell receptors that have an alpha and a beta polypeptide subunit. And we also have gamma delta T cells which contain T cell receptors that consist of a gamma and a delta polypeptide subunit. Now, it turns out that we know much more information about AlphaBeta T cells than we do about gamma delta T cells. And that's exactly why in this lecture we're going to focus primarily on alpha beta T cells. Now, before we take a look at these alpha beta T cells let's recall what an antigen presenting cell is or an APC.