Final answer:
The initial immune response that occurs after flea bites in flea allergic dermatitis is called immediate hypersensitivity or type I hypersensitivity.
Step-by-step explanation:
The type of immune response that develops initially after flea bites in the case of flea allergic dermatitis (FAD) is called immediate hypersensitivity or type I hypersensitivity. This is an allergic reaction that occurs rapidly, within minutes of exposure to the flea antigen. The immune response involves the production of IgE antibodies that bind to mast cells, and upon subsequent exposure to the antigen, the mast cells release chemical mediators such as histamine and serotonin, leading to the typical allergic symptoms such as itching and inflammation.