What are natural killer cells?

What proportion of lymphocytes in peripheral blood do natural killer cells constitute?

Which cells do natural killer cells display cytotoxic activity against?

Natural killer cells do not recognise tumour cells or virally infected cells by antigen as they do not have toll-like receptors or immunoglobulin in their cell membranes. How do they recognise their potential targets?

What do natural killer cells look like when attack infected/tumour cells?