Extra

FMC7

The FMC7 antibody recognises an epitope of CD20 that has a more restricted pattern of expression than the parent molecule. FMC7 has high expression on late-stage b-cell neoplasms such as hairy-cell leukaemia or MZL and can be used to indicate their possible presence; however low level of FMC7 expression is frequent on other B-cell disorders so care is required in interpretation.

What is its normal function and expression

The identity of the FMC7 antigen was unclear for many years before being identified as an epitope of the CD20 molecule. FMC7 never received a CD classification number and retains the initials of the laboratory where it was raised: the Flinder Medical Centre. The particular function of FMC7 is not known, but like the CD20 molecule the FMC7 antigen is expressed by normal B-lymphocytes from the late pre-B-cell stage of development until their terminal differentiation and so is effectively a pan b-cell antigen that is particularly associated with late B cells that have features of activation.

What is its diagnostic role

• FMC7 is widely expressed in B-NHL particularly where there is strong CD20 expression.
• CD20 tends to have low expression on CLL but higher expression on other B-cell types particularly HCL, MCL and MZL
• FMC7 therefore is most often used as an alert to draw attention to such disorders

Summary tables

1. expression by acute leukaemias and by haematogones rare pro-b all rare early pre-b all rare pre-b all hi b-all rare pro-t all rare mature-t all rare aml poss haemato-gones

2. expression by b-lymphoproliferative disorders

poss cll * hi pll** hi mcl freq fl hi hcl** hi hclv ** hi mzl hi lpl poss pcs