Final answer:
Cytokines are the main alert system for white blood cells, signaling them to react against invaders. Antigens are foreign substances that provoke a response, while antibodies produced by B cells specifically target antigens and can activate other immune responses.
Step-by-step explanation:
The pieces of the immune system that alert white blood cells to an invader's presence are cytokines. Cytokines are small proteins released by cells, especially those in the immune system, with a specific effect on cell-to-cell communication. They signal the immune system to do its job, like calling white blood cells to the site of an infection, inflammation, or trauma. In contrast, antigens are substances that can cause an immune response, and the immune system recognizes them as foreign. Antibodies, on the other hand, are proteins produced by B cells that specifically target antigens for neutralization or destruction. They also play a role in alerting white blood cells and can activate the complement system, which assists in destroying pathogens.