Question

"each lymphocyte has thousands of receptor proteins in its membranes. what is the job of these receptor proteins?"

Answer 1

Some of these receptors bind to antigens in the blood. The antigen are then endocytosed and identified.  Some of the receptors such as the MHC II are used in antigen-presentation. They display the antigen for identification and activation of other lymphocytes.

Answer 2

A receptor is in the cell membrane. The lymphocytes having thousands of receptor in the membranes are then use to recognize antigens which is the answer. Antigens are the molecules being recognize by the T and B cells. Hope this is the answer and would be of help then.