Extra

A pan B-cell marker with good specificity for the identification of B-lineage. CD79a is expressed initially in cytoplasm of early B cells before appearing on the membrane at the time heavy chains of immunoglobulin become expressed.

normal expression and function

CD79a associates with CD79b and the immunoglobulin chains forming the B-cell receptor. These chains mediate intracellular signalling. However the two chains are expressed slightly differently - CD79a is a more lineage-specific marker than CD79b which often has more variable expression. Expression is acquired at a very early stage of B-lymphocyte formation - initially in cytoplasm, then appearing on the membrane of mature cells and persisting until plasma cell differentiation when it is progressively lost.

diagnostic role

• CD79A is a reliable marker of B-lymphocyte lineage with low levels of aberrant expression; it is more specific than CD19 or CD20 although it may rarely be expressed on other cell types.
• in ALL expression of CD79A in cytoplasm is acquired at an early stage of B-lineage commitment while expression is on the cell surface occurs at later differentiation at the time surface heavy chains are acquired.
• in myeloma about half of cases will express CD79A strongly, but a further proportion have weak expression.

other relevant information

CD79a expression is recognised on some cases of AML and T-ALL so caution should be exercised if other features are atypical. Like CD19 the expression of CD79a by AML is more often associated with the t(8;21) translocation.

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summary tables 1. expression by acute leukaemias and by haematogones file freq pro-b all hi early pre-b all hi pre-b all hi early b all rare pro-t all rare mature-t all poss aml hi haemato-gones link flow cytometry key CD79a cytoplasmic together with CD19 and CD22 can be used in to indicate b-lineage in pro-all and remains cytoplasmic until the b-all stage 2. expression by b-lymphoproliferative disorders file hi frame cll * hi pll** hi mcl hi fl * hi hcl** hi hclv hi mzl hi lpl freq pcs