Monocyte monolayer assay

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The Monocyte Monolayer Assay (MMA) helps decide if antibodies in a transfusion patient could cause trouble. It checks whether alloantibodies—antibodies directed against donor red blood cell antigens—could lead to destruction of donor cells after a transfusion.

How it works in simple terms: if a donor red blood cell has an antigen the patient’s body sees as foreign, the patient may make alloantibodies against it. Monocytes, a type of white blood cell, can swallow foreign material. In the MMA, monocytes are exposed to antibody-coated donor red cells. If the monocytes engulf these cells (phagocytose them), the antibodies are considered clinically significant because they could cause the donor cells to be destroyed in the body.

Why this matters in blood banking: labs routinely test blood to determine blood type, screen for antibodies, and match compatible blood. Many antibodies to non-ABO antigens are possible, and finding out which are truly dangerous can be hard. Techniques like elutions, adsorptions, and enzyme treatments are sometimes used to identify these antibodies. Some antibodies target high-frequency antigens found on many people’s red cells, which makes interpretation tougher. The MMA helps decide if such antibodies are clinically important.

What makes an antibody clinically significant: it can reduce donor red cell survival or cause hemolysis (destruction of red cells) after transfusion. Hemolytic transfusion reactions can be acute (within 24 hours) or delayed (up to 28 days). ABO antibodies behave according to Landsteiner’s law: people with A or B antigens don’t have antibodies against those antigens, while people lacking both A and B can have anti-A and anti-B antibodies. In total, about 47 blood group systems with many antigens exist, and most antibodies arise after exposure to an antigen through transfusion, stem cell transplant, or pregnancy.

Key features of clinically significant alloantibodies: they are usually IgG, react at body temperature, and cause red cell clumping in an indirect antiglobulin test. Sometimes antibodies to common antigens hide other, clinically important antibodies, which is where the MMA can help.

The MMA is a manual, lab-based test. A positive result—often defined as more than about 20% of donor red cells being phagocytosed by monocytes—suggests the patient’s serum contains clinically significant antibodies likely to cause problems with transfused cells. If the test results are positive, clinicians may choose blood products that are less likely to react with the patient’s antibodies.