Several tests to detect
antibodies to platelets in patients with
immune thrombocytopenia have been developed over the 40 years since it was first noted that this disorder was mediated by antiplatelet factors. Early tests were crude and not reproducible. A major landmark was the quantitation of
IgG immunoglobulin on platelet membrane and the observation by several workers that marked increases of all classes of
immunoglobulin occurred on platelets in patients with
immune thrombocytopenia. Although at one time accepted as a diagnostic characteristic of
autoimmune thrombocytopenia (AITP), the initial euphoria was tempered over the last decade by the realisation that elevation of platelet associated
immunoproteins was not pathognomonic of this disorder and that raised levels were seen in several other disease processes, sometimes even when platelet counts were normal. The nature of these
immunoproteins needs careful understanding. True platelet
autoantibodies will manifest as increased platelet
immunoproteins but not all such platelet
proteins are platelet
antibodies. There is speculation about the existence and the mode of activity of
IgA and
IgM immunoglobulins, both commonly found on platelets in AITP. It is sometimes almost inconceivable that the extremely large amounts of PAIgG could possibly represent true
autoantibody.
Immune complexes are found in plenty in these and other disorders in which
thrombocytopenia manifest. In such situations it is likely that 'amplified'
immune complexes are bound by
Fc receptors, as may be found in viral mediated
ITP or in septicaemic states. There is now sound evidence that several
glycoprotein such as
GP IIb/IIIa, GP Ib, GP V, found on platelet membrane, act as target
antigen sites for the attachment of
antibodies to platelets.(ABSTRACT TRUNCATED AT 250 WORDS)