Alloimmune memory is absent in Hymeniacidon sinapium, a marine sponge.

The southern California sponge Hymeniacidon sinapium can recognize and reject transplanted allogeneic tissues. This species reveals a complex rejection response that involves cellular infiltration into the graft interface and a deposition at that site of a fibrous wall. The response then proceeds to a cytotoxic reaction in which the cells in the graft zone become necrotic and slough off, resulting in allogeneic tissue separation. The rate and intensity of this response varies with the genetic constitutions of the paired sponges and the temperature of the water. In experiments employing pre-sensitized second-set and unrelated third-party graftings, with changes in temperature, sensitization times, intervals before regrafting, and experimental sites, this species did not exhibit alloimmune memory (i.e., accelerated second-set rejection). When the allorejection described here for H. sinapium is taken in conjunction with rejection responses reported for other sponge species, it is apparent that sponges have two major methods for responding to allogeneic contacts: barrier formation or cytotoxicity. The rejection method seems to correspond with the presence or absence of immune memory, and may, in part, be correlated with habitat and frequency of contacts with "non-self." The development of the rudimentary immune system in metazoans is discussed in relation to the rejection of naturally transplanted tissues in addition to challenges from pathogens.