Immunophenotyping is a process that analyses the types of markers or antigens on the surface of the leukaemia cells. It is used to diagnose specific types of leukaemia by comparing the leukaemia cells to normal cells.

Immunophenotyping involves the use of a device called a flow cytometer. A blood sample containing leukaemia cells is suspended in a fluid, and run through the flow cytometer. It rapidly analyses and quantifies the types of antigens on the surface of the leukaemia cells.

Diagnosis is usually possible by performing immunophenotyping of peripheral blood only. However, if the immunophenotyping is not conclusive, a bone marrow or lymph node biopsy can be helpful.

CLL leukaemia cells show the B-cell surface antigens CD19 and CD20, as well as CD5, CD23, CD43, CD79b and CD200.

The levels of CD20 and CD79b on the leukaemia B-cells are routinely much lower compared with those on normal B-cells.