The question was, "is it possible to have something resembling a human immune system without a full body?" The answer is no, for somewhat obvious reasons. The

The question was, "is it possible to have something resembling a human immune system without a full body?" The answer is no, for somewhat obvious reasons. The human immune system involves many organs and tissues and cells working together in very complex ways. And I think your charitable interpretation is quite a bit too charitable. OP did not say anything like "why doesn't it work like I think it would".