At least that's the current thinking based on our fractious knowledge of natural history. For our purposes we could expect that the vertebrate immune system should develop from the rudimentary in fish to the most complex in ourselves. Invertebrates exhibit a variety of defense mechanisms. All vertebrates however, have generally the same immune system which has developed in subtly different ways for different species. The vertebrate humoral immune system Antibodies are the defense of choice in all vertebrates.
Antibodies are structurally unrelated to the humoral factors found in invertebrates.
They have much more flexibility and are much more potent in their defense ability. In humans there are several different Subtypes of antibodies with different functions. We will look at this later. We can see an advance in complexity of response from simple to complex vertebrates. Simple vertebrate agnathan fish do have a very primitive form of antibody.