We have previously shown that specialised, antibody-producing cells of the immune system, called B-cells, migrate from the draining lymph node into the breast tumour, where they are stimulated to multiply and mutate their antibody genes. Capitalising on these discoveries, we are cloning the antibody genes from B- cells specific for both known and unknown tumour-associated molecules recognised by the immune system (antigens). Using genetic engineering methods, we are reconstructing the original antibodies made by B-cells infiltrating breast tumours and draining lymph nodes from our cloned antibody genes. These reconstructed antibodies are being used to identify and characterise tumour antigens recognised by the immune system and to determine the effects of mutations in the antibody genes on the ability of the immune system to kill tumour cells. This will enable us to identify novel tumour antigens recognised by the immune system and to develop therapeutic human antibodies and an effective tumour vaccine.