Can fetal antigens be used for prophylactic immunization?

Even if the inoculation of fetal tissue cells were a dependable and uniformly successful method of protecting experimental animals against chemically or virally induced tumours, it would for obvious reasons still not be feasible to use fetal tissues for such a purpose in human beings. Among possible substitutes syngeneic spermatozoa were tested on the grounds that they are the only adult cells that express T-alleles, but neither they nor teratocarcinoma cells protected mice against tumours raised by 3-methylcholanthrene. Testicular and thymic cells and tissue fragments have given effective protection in a number of experiments and it is noteworthy that fetal tissues, testicular cells and thymus cells are cross-reactive in respect of anti-embryo antibodies. Testicular cells probably act like fetal cells and, like fetal cells, are very prone to give rise to 'enhancement'. Thymic cells do not 'enhance' and may act quite differently. The variability of results--a source of grave concern--is attributed to the insensitivity of the test system which is ill-adapted to show up low degrees of protection.