Final answer:
Lymphocytes are the leukocytes involved in antibody production and cellular immunity, specifically through the actions of B and T cells.
Step-by-step explanation:
The leukocyte that is involved in antibody production and cellular immunity is the lymphocyte (B) Lymphocyte). Lymphocytes, which include B cells and T cells, are responsible for the adaptive immune response within the body. B cells play a crucial role in antibody production when they mature into plasma cells, while T cells are mainly associated with cellular adaptive immunity. The other leukocytes listed have different functions: Neutrophils are phagocytic defenders against bacterial infections, monocytes develop into macrophages and dendritic cells that engage in phagocytosis and promote inflammation, and eosinophils combat parasitic infections and participate in allergic responses.