2004). B-lymphocytes, the cells that produce antibodies, are initially produced in the embryonic liver, yolk sac and bone marrow, then move through the blood to the bursa of Fabricius (BF). Birds have a bursa of Fabricius (BF), which is an outpocketing of cloaca.
Most squirrels are vegetarians. There are a few exceptions like the flying squirrel. The flying squirrel will eat small birds, insects and definitely steal eggs from birds nest in order to make a meal. Generally speaking though, squirrels will mainly stick to things that are vegetarian related.
Even if a squirrel finds poison, it eats it rarely. This especially refers to rat poison. After everything you have read it probably seems impossible to kill them. Still there are a few options: Rat Poison. Even if it kills them, they will remain in your house and will start smelling. Anticoagulants.