Immunofluorescence light micrograph showing CD4+ (green) and CD8+ (yellow) T cells (tumour-fighting immune cells) in the microenvironment of a squamous cell carcinoma (SCC), the second most common form of skin cancer. SCC is characterised by the accelerated growth of squamous cells, which are found in the tissue that forms the surface of the skin. SCC occurs when DNA (deoxyribonucleic acid) damage from exposure to ultraviolet radiation or other damaging agents trigger abnormal changes in squamous cells.