Tumour immunology

A tumour (neoplasm) is a mass of cells whose growth has escaped normal control. When such a tumour keeps growing and invades surrounding healthy tissue, it is called cancer; cancer cells that spread from the original tumour to establish new growths elsewhere in the body produce secondary tumours, a process called metastasis. Tumours are classed as benign (capable of indefinite growth but not invading other body parts) or malignant (growing indefinitely, detaching, and migrating into surrounding healthy tissue). The underlying defect is a breakdown of the tight regulation that normally governs cell growth and differentiation: healthy cells display contact inhibition, which halts uncontrolled proliferation once cells touch their neighbours, but cancer cells lose this property and divide continuously, building up tumour masses.

When a cell undergoes malignant transformation, it typically gains new surface (tumour) antigens and may lose some of its normal ones; because these altered antigens sit on the malignant cell's membrane, they can provoke an immune response — both humoral and cell-mediated responses can be seen against malignancy. Cancer cells are nonetheless hard to eliminate immunologically because, unlike a foreign pathogen, they are the body's own abnormally functioning cells rather than a genuinely foreign body, letting many of them evade immune detection. The concept of immunological surveillance holds that one core job of the immune system is to continually seek out and destroy malignant cells that arise spontaneously through somatic mutation; when this surveillance weakens — through ageing, or through congenital or acquired immunodeficiency — cancer incidence rises, meaning the very emergence of a tumour represents a lapse in that surveillance. …