evolution - How and when did a dedicated immune system evolve?

I have recently been doing a lot of research into the interplay between the innate and adaptive immune systems in humans, and mammalian laboratory models. This has led to my reading some interesting information on the immune response in insects;

Insects have a highly efficient immune system. In response to a bacterial attack, their fat body (the equivalent of the liver in mammals) synthesizes a whole range of peptides with an antibacterial and antifungal effect.

This fascinated me, as the clear inference is that there are no ‘dedicated’ immune cells, but that adipose tissue has far more diverse functions that I had realized.

I have done a little more reading, and also looked at plant immune systems, which seems far more analogous to those in insects than mammals;

Plants, unlike mammals, lack mobile defender cells and a somatic adaptive immune system. Instead, they rely on the innate immunity of each cell and on systemic signals emanating from infection sites.
(Jones, 2006)

My questions relates to the need of an adaptive immune response in mammals. The immune systems in insects and plants - a more 'systemic' immunity due to the lack of dedicated/mobile immune cells - seems much simpler.

Given that evolution works incrementally (there are no 'jumps' - for instance, going from a non-dedicated immune system, to a dedicated immune system), I would hypothesise that organisms less distantly related to insects and plants may have tissues with duel functions (similar to insects?), but that specialize further as immune cells until gradually (down the evolutionary tree) a complex and specific immune system emerges. (This is complicated by the fact that our immune systems do have multiple roles - e.g. tissue remodelling, but I wasn't going to go into that here. Feel free in your answers if it is necessary!).

My overall curiosity can be summarized as 2 questions;

1. What are the possible reasons why a dedicated and immensely complex immune system evolved in some lineages of organism?


2. Is there any evidence of 'half-way' organisms, and about what ecological time-frame might the dedicated immune system have developed?