Sort of? The immunity that a chick has initially isn't its own, but passively transfered via the egg from the hen ("maternal antibodies"). They're much shorter-lived than mammalian ones, so these initial antibodies are pretty much undetectable by two weeks old. Increasing exposure to pathogens during this period has both good and bad consequences: Good because since the maternal antibodies will "block" most of the pathogens, only a very small number will get through to the chick's own immune system, where they can be grabbed by macrophages and taken to the B-cells to "learn" the pathogen and start the process of making its own antibodies against it. Bad because those maternal antibodies aren't replenishable, so if the challenge is too high all of them will be used up and the chick will be vulnerable to disease (there is a lag between the presentation of the antigen to the B-cell and the initial production of antibodies...roughly 5 days or so).
And here's another wrinkle that I just learned about! The B-cells originate in an organ called the Bursa of Fabricius. When the chick hatches, these B-cells are undifferentiated...basically blank slates. While in the bursa, they undergo a process called gene conversion to produce the diversity of antibody-producing types needed to protect the chick. This can happen without the stimulation of outside pathogen presentation, but it boosts the diversity TREMENDOUSLY if these cells are exposed to external antigens. Two critical points of timing for this: you get the most diversity before the chick is 6-10 weeks old, and the bursa basically closes up shop around 6 months, sending any remaining B-cells out into the body.
The upshot is (and returning to the original thread topic) that giving them the chunk of sod before they are 6 weeks old is a good idea immunologically. There may be special protections for doing it before 2 weeks old, but you may open them up to other things so...six of one, half-a-dozen of the other...
