Can you tell me what is an immune system?

Keeps us healthy: Pretty big subject for this venue. Humans have an advanced immune system that has many facets, including aspects that are innate (born with) and adaptive (learned from exposure to the environment). Immune cells (white blood cells, lymph nodes, bone marrow, spleen, liver) antibodies, proteins (including complement), that function with heirarchy, transport, communication, and redundancy.