Immunity is caused by

Immunity is caused by Lymphocytes.
A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates.
Lymphocytes function to eliminate the antigen, either by releasing antibodies (in the case of B cells), cytotoxic granules (cytotoxic T cells) or by signaling to other cells of the immune system (helper T cells).
Lymphocytes help your body's immune system fight cancer and foreign viruses and bacteria (antigens). Lymphocytes help your immune system remember every antigen it comes in contact with. After an encounter, some lymphocytes turn into memory cells.