The existance of these cells is a relatively recent discovery and hence their functioning is still somewhat debated. The basic concept of suppressor T-cells is a cell-type that specifically suppresses the action of other cells in the immune system, notably B-cells and T-cells , thereby preventing the establishment of an immune response. How this is done is not known with certainty, but it seems that certain specific antigens can stimulate the activation of the suppressor T-cells. Discrete epitopes have been found, that display suppressor activity on killer T-cells , T-helper cells and B-cells . This suppressor effection is thought to be mediated by some inhibitory factor, secreted by suppressor T-cells. It is not any of the known lymphokines . A fact that renders the study of this cell-type difficult is the lack of a specific surface marker. Most suppressor T-cells are CD8 positive - as are cytotoxic T-cells .