Extra

Lght chains form part of the B-cell receptor and may therefore be considered as a pan B-cell marker that is expressed from the late lymphoblast stage until plasma cell differentiation begins. Where cells express predomnantly kappa or lambda (light chain restriction) then clonality can be inferred.

normal function and expression

The immunoglobulin molecule (with either kappa or lambda light chain) forms the antigen-recognising portion of the B-cell receptor. Normal mature B cells therefore express light chains which are formed during pro-B to pre-B cell development appearing at the surface and remaining expressed on the cell surface until plasma cell development (when the immunoglobulin molecules become internal contaned within cytoplasmic secretory vesicles).

what is its diagnostic role

• Indication of stage: acquisition of surface light-chain expression does not occur until a late point in B-precursor cell development so is useful be used to indicate developmental stage in ALL. Similarly, surface expression is lost at the point of plasma cell differentiation so absence od surface expression can indicate plasma cell developmental stage.
• Indication of clonality: malignant b-cells arise from a single parent cell so express light chain of a single type (either kappa or lambda) - light chain restriction. In many cases some normal cells will also be present so light chains may be a mixture of kappa and lambda. However when there is a malignant clone, cells expressing either kappa or lambda will predominate. Studies most frequently suggest that if the kappa to lambda ratio is greater than 3 1 then the presence of a clonal population can be inferred (although correlation with other markers is needed).
• Weak or absent light chain expression may be seen in DLBCL FL or CLL and more rarely in MCL or MZL: overall around 10% of B-lymphoproliferative disorders do not express surface light chain.

summary tables

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

• expressed by late haematogones

2. expression by b-lymphoproliferative disorders hi cll hi pll** hi mcl hi fl hi hcl** hi hclv l hi mzl hi lpl poss pcs