 A T cell receptor that binds mostly to MHC class I tends to produce a mature "cytotoxic" CD8 positive T cell; a T cell receptor that binds mostly to MHC class II tends to produces a CD4 positive T cell.T cells that attack the body's own proteins are eliminated in the thymus, called "negative selection".Epithelial cells in the medulla and dendritic cells in the thymus express major proteins from elsewhere in the body.