Think of primary lymphoid organs as the "schools" where immune cells learn to recognize enemies, and secondary organs as the "battlefields" where they actually fight them.
Step 1: According to the passage, primary lymphoid organs serve as the "training ground" for immune cells. Step 2: In these organs, immature lymphocytes undergo genetic rearrangement and maturation. Step 3: The result is the production of antigen-sensitive lymphocytes, which are capable of recognizing specific foreign invaders but have not yet encountered them.