This occurs when T and/or B clones of lymphocytes are depleted in size or suppressed in their reactivity, expansion or differentiation. 
It may arise from activation of specific or nonspecific T suppressor lymphocytes of either T or B clones or by drugs that have generalised effects on most or all T or B lymphocytes. 
Cyclosporin A and FK506 act on T-cells, as does antilymphocyte serum, alkylating agents such as cyclophosphamide are less specific in their action and damage DNA replication, while base analogues interfering with guanine metabolism act in a similar way.