CD23 is the low affinity receptor for IgE. When it appears on eosinophils and macrophages it helps these cells to kill and eat structures that are coated with IgE antibody, but this isn't what it does on CLL cells. Some have suggested that it is a growth factor for B cells, but this is disputed. Others have suggesetd it binds IgE immune complexes and this increases the efficiency of B cell antigen presentation and processing. I can't see why this is important for CLL cells. We also know that it is associated with the fyn tyrosine kinase.
It is present on CLL cells, but not usually on other types of B cell lymphoma cells. It is also present in the serum of CLL patients. In continental Europe, particularly Italy, it has been used as a prognostic factor for CLL. It has not traveled. I think the main problem with it is that it is a surrogate marker for both the amount of CLL cells as well as their growth rate, and therefore not particularly useful for predicting the future.