Heterophile antibodies are antibodies induced by external antigens (heterophile antigens).
Some cross-react with self-antigens. For example, in rheumatic fever, antibodies against group A streptococcal cell walls can also react with (and thus damage) human heart tissues. These are considered heterophile antibodies.
Heterophile antibodies can cause significant interference in any immunoassay (note: an immunoassay is a biochemical test- and one frequently used in medical diagnostic testing- that measures the presence or concentration of a macromolecule in a solution through the use of an antibody or immunoglobulin. The presence of a heterophile antibody is characterized by broad reactivity with antibodies of other animal species (which are often the source of the assay antibodies). They are commonly referred to as humananti-animal antibodies (HAAA). Human anti-mouse antibodies (HAMA) belong to this category. They can create both false positive and false negative results. 
So-called 'sandwich' immunoassays are particularly susceptible to this interference. (Sandwich immunoassay = two-site, noncompetitive immunoassays in which the analyte in the unknown sample is bound to the antibody site, then labeled antibody is bound to the analyte. The amount of labeled antibody on the site is then measured. It will be directly proportional to the concentration of the analyte because labeled antibody will not bind if the analyte is not present in the unknown sample. This type is also known as sandwich assay as the analyte is "sandwiched" between two antibodies.) Heterophile antibodies may thus give false positives (by bridging the capture and signal antibody) or false negatives (by blocking one or the other). Both detecting and deterring this interference is difficult in clinical medicine. One option is to repeat the test using a different type of assay. Other options include the use of heterophile blocking reagents, steps to remove immunoglobulins, and serial dilutions.
Heterophile antibody interference does not change linearly with serial dilution, but a true result will. This is one strategy for heterophile antibody detection.
Blocking heterophile antibody interference can be achieved by removal of immunoglobulins from a sample (such as with PEG), by modifying antibodies which may be present in a sample or by using buffers to reduce interference.
Heterophile antibodies are of particular importance in clinical medicine for their use in detecting Epstein-Barr Virus (the causative agent of infectious mononucleosis). EBV infection induces the production of several antibody classes, of which heterophile antibodies are one (others include anti-i, rheumatoid factor and ANA). Heterophhile antibodies are IgM antibodies with affinity for sheep and horse red blood cells. They appear during the first week of infectious mononucleosis symptoms, 3-4 weeks after infection and return to undetectable levels 3 to 6 months after infection.
Heterophile antibody is a fairly specific but insensitive test for EBV. It is present in 80% of infected teens and adults, 40% of all infected children, and only 20% of infected children under 4. Heterophile antibodies can arise in non-EBV infections. False positive monospot tests may occur in cases of HIV, lymphoma or lupus. Other assays for detection of EBV are available, including serologic markers. 
- The Ghost in the assay tube: heterophil antibody interferences in immunoassays – an ever-recurring but often forgotten problem. Communiqué: A Mayo Reference Services Publication. 2003; 28(3):1–4.
- ASCP Quick Compendium of Clinical Pathology, 2nd Ed. Daniel De Mais. ASCP Press 2009.