A 'neutron bomb' antibody, which kills cancer cells but leaves healthy ones unscathed, could be used to treat the disease.
Researchers have developed an antibody from the body's own immune system that homes in on cancer cells.
The antibody targets a natural defence mechanism that cancer tumours exploit.
Special proteins guard the surface of cells to prevent the body's own immune system attacking them.
Researchers say they are "excited" by the findings, which could provide a whole new way of treating cancer.
In a study published in Cell Reports, researchers developed and tested the cancer-fighting antibody.
The human-derived antibody dismantles a specific part of the cancer cell's defence system, before launching an attack.
Scientists began their research after observing that some lung cancer patients have early-stage tumours that never progress to advanced stages.
The patients with the slower-progressing tumours had antibodies against a specific protein, called complement factor H, or CFH, which protects cells from an immune system attack.
CFH prevents an important immune system response from activating. It does this by preventing a deposit of complement C3b protein on the cell surface.
When complement C3b reaches the cell, it can cause the membrane to deteriorate and, ultimately, the cell to die.
After identifying the antibody for CFH, researchers from Duke University, North Carolina, then sought to find a way of producing antibodies that recognised the same part of the CFH as the autoantibodies made by the early-stage cancer patients.
The researchers then pooled the white blood cells from the CFH antibody-producing cancer patients, before isolating and cloning the antibody genes from single immune cells.
These mature antibodies recognised the same region of CFH targeted by the original patient's immune systems, so healthy cells were left unharmed.
The team went on to test the new treatment on multiple cancer cell lines, including lung, gastric and breast cancers, both in lab dishes and on living mice.
Author Dr Edward Patz, from Duke University, said: "This is the first completely human-derived antibody developed as an anti-cancer therapy, which is very different from other immunotherapy approaches.
"We believe it might be this additional cellular response that could potentially have the most profound impact on cancer outcomes long-term."
While researchers say further tests are needed, the team are excited about the new findings.
Dr Patz added: "This could represent a whole new approach to treating cancer, and it's exciting because the antibody selectively kills tumour cells, so we don't have significant side effects to achieve tumour control.
"We believe we can modulate the immune response and let the body's own immune system take over to either kill the tumour or keep it from growing."