Amensalism differs from parasitism in that:
Amensalism has the sign pattern (−, 0): one species is inhibited or damaged while the other has no measurable net response. Parasitism has the pattern (−, +): the parasite benefits by obtaining resources or reproductive access from a host that is harmed. Therefore, an unaffected participant distinguishes the simplified amensal interaction. Antibiosis illustrates the idea when a microorganism releases a compound that suppresses another species without an evident benefit or cost to the producer. By contrast, a parasite’s nutrition, shelter, or transmission depends on exploiting its host. Both species do not benefit in either interaction, and both being harmed would indicate competition. Nutrient acquisition by one participant is characteristic of parasitism, not amensalism. Real systems can be difficult to classify because suppressing a neighbor may indirectly release resources and benefit the inhibitor, converting apparent amensalism into interference competition. The sign notation should therefore be based on measured changes in fitness under defined conditions, with “unaffected” meaning no detectable net effect rather than absolute absence of any physiological interaction.
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 8-13