Monocytes are the largest type of white blood cell and they play a key role in the immune system by:

* Phagocytosis: They engulf and destroy bacteria, viruses, and other foreign invaders.

* Antigen presentation: They present antigens (parts of foreign invaders) to other immune cells, helping to trigger an immune response.

* Tissue repair: They contribute to tissue repair and regeneration.

While other white blood cells like lymphocytes also have a large nucleus, they are distinguished by their smaller size and different functions.