Are positive and negative selection unique to T cells?

Yes, positive and negative selection are unique to T cells. These two processes take place in the thymus as part of the development of T cells. 

Positive selection occurs first. This process ensures that T cells are able to recognize a wide variety of foreign invaders, such as bacteria and viruses.

Negative selection, which is the second phase of the selection process, prevents the development of T cells that might recognize and attack the body's own healthy cells, causing autoimmune diseases. This process ensures that only T cells with an appropriate response to foreign invaders are allowed to mature and pass through to the bloodstream.  

Other cells in the body do not need this type of specialization and hence do not need to undergo positive and negative selection.