FMC7

Summary

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.

Normal expression and function

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.

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

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SUMMARY TABLES

Note The colour and % refer to the proportion of cases expressing the marker. Additional comments on strength, or site of expression (if relevant) are given below the table. For full details of key click here. For further information about a particular disease state click <I> adjacent to the condition name

Expression in primitive cell types
AML	CMML	B ALL	BL	T ALL	ETP ALL	MPAL	Hgn
5-20%	<5%	20-40%	40-80%	5-20%	<5%	20-40%	5-20%

Notes: Although not generally tested in these disorders the expression patterns tends to resemble CD20

Expression in mature B cell neoplasms
CLL	MCL	FL	MZL	HCL	DLBCL	LPL	PCL
20-40%*	40-80%	20-40%*	40-80%	80-190%	20-40%*	40-80%	<5%

Notes: * Expression is less frequent and often is weak in these disorders

Expression in mature T cell neoplasms
ATLL	CTCL/Sezary	T-PLL	T-LGL	NK-LGL
<5%	<5%	<5%	<5%	<5%

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