Perhaps you could read my comment again. How can a creature develop an immune system if it does not know what a threat is. By the time it knows (a great stretch to imagine a sentient blob, I know) that it needs an immune system, it is dead. Oh, I suppose that it can evolve while it is dead, if you stick with your logic.
your understanding of the sitaution is lacking making you not understand it very well.
the first multicellular forms would have been colonies, like with jellyfish the closest it has to a immune system is that it's body may fend off bacteria and such, the first life forms would have simple, and made of protozoa that could combat bacteria naturally, as they got more complex, more complex features would form, but at first it wouldn't be very complicated or the bacteria attacks very complicated either
Remember one of the key things in evolution is variation, how does a bacrteria know to be resistant against a anti bacteria agent it's never seen? It doesn't, but the mutations with a colony of bacteria would be random enough that some might be more able to defend against it then others. Same with a colony, they had already been fighting bacteria and such for billions of years, being multi celluar doesn't mean they suddenly forget how that works. Plus having a immune system would put selective pressure on the animals to evolve some defense. At first when it appeared it probably be a random mutation that seems to be nothing, but when it deals with bacteria it suddenly is more useful. The creationist problem is they think it's all or nothing, or that the solution must come at the same time as the problem. It can come later or sooner.
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