Concept:
Antibodies are proteins produced by B-lymphocytes (a type of white blood cell) to protect the body against harmful pathogens.
Step 1: Identify the function of each cell.
• Erythrocytes -- Transport oxygen.
• Monocytes -- Engulf pathogens by phagocytosis.
• Granulocytes -- Defend against infections.
• Lymphocytes -- Produce antibodies and provide immunity. \checkmark
Step 2: Final conclusion.
Therefore, antibodies are produced by
\[
\boxed{\text{Lymphocytes}.}
\]