The Immune System is built on millions of cells called

White blood cells: lymphocytes, neutrophils, and monocytes

Explanation:

Lymphocytes are processed in the bone marrow and then migrate to the lymphoid organs. The cells of the immune system are white blood cells: lymphocytes, neutrophils, and monocytes.

The immune system comprises of many biological structures and processes within an organism that provides resistance to infection and toxins.