The effect of cyclophosphamide on immunological control mechanisms.

Cyclophosphamide (CY) given three days before immunization with a range of antigens increases the intensity of cell-mediated immune responses. CY given around the time of immunization also reverses immunological tolerance, blocks antigenic competition and modifies desensitization. The effect on antibody production is not so predictable. However, it may increase the IgE response without affecting IgG1 or IgG2 response to the same antigen and may abolish immunological memory. The action of CY is on rapidly-turning-over cell populations. Lymphocytes are maximally at risk 48 hours before they transform into large pyroninophilic cells, that is just before the S phase of cell division. CY has a preferential effect on both T- and B- suppressor cells. The application of these findings to experimental cancer models is discussed. These findings are not restricted to CY, as a wide range of cytotoxic drugs with different actions at the cellular level may have similar profound effects on the regulation of the immune response.