(Immune Destruction of Blood Platelets)
Arrows point to two platelets amid a group of red blood blood cells
WHAT IS A PLATELET?
A platelet is a cloud-shaped blood cell, neither related to the red blood cell line nor the white blood cell line. Platelets assist in the clotting of blood in that they home to damaged areas of blood vessels, and “aggregate” there, meaning that they pile onto each other and bind, forming a small plug to seal the hole in the leaking blood vessel. While piled on each other, they release assorted biochemicals initiating a more permanent fibrous seal of the tear. Of course, large tears are too big for platelets to seal but when it comes to small bleeds and normal blood vessel wear and tear, platelets are the star of the show.
There is a saying that “platelets are vascular integrity and vascular integrity is platelets.”
A small bleed unstaunched by a platelet aggregation quickly becomes a large bruise. Spontaneous bruising (in other words visible bruising from the normal wear and tear of one’s body) is a sign of reduced platelet numbers or poor platelet function.
THE LIFE AND TIMES OF JOE PLATELET
IMMUNE-MEDIATED PLATELET DESTRUCTION
For reasons unknown, platelets can be mistaken by the immune system as invaders. When this happens, antibodies coat the platelets and the spleen’s phagocytes remove them in numbers up to 10 times greater than the normal platelet removal rate. The megakaryocytes in the bone marrow respond by getting larger and growing in numbers so that they may increase their production of platelets. The platelets produced under these circumstances tend to be larger and more effective than normal platelets and are called “stress platelets.” The bone marrow attempts to overcome the accelerated platelet destruction rate; unfortunately, with immune-mediated destruction occuring, a human platelet can expect to survive only one day in the circulation instead of its normal 6-8 days. If antibody levels are very high, a platelet may survive only minutes or hours after its release from the bone marrow and, making matters worse, antibody coated platelets still circulating do not function normally. This is balanced by the especially effective stress platelets entering the scene so that overall it is hard to predict how the balance will work out in a given patient.
WHAT WOULD CAUSE THE IMMUNE SYSTEM TO GET SO CONFUSED?
In many cases, a cause is never found; however, in cases a primary reaction in the immune system precedes the platelet destruction. Keep in mind that the immune system responds to the shapes of proteins present on a cell’s surface. These shapes are similar to ID cards. The immune system recognizes shapes defined as “self” and does not attack but when it sees a cell expressing protein shapes that are “non-self,” it will respond.
If the immune system is responding to a blood parasite, tumor, drug, or other cell type (as in lupus or immune-mediated red cell destruction), it will be producing antibodies against “enemy” shapes. Some of these shapes may, unfortunately, resemble some “self” shapes such as some of the shapes on the surface of the platelets. The platelets are then mis-identified as the enemy and are attacked. Alternatively, foreign proteins may actually stick to the platelet surface thus fooling the immune system to respond to the platelet as a whole.
OTHER CAUSES OF PLATELET DYSFUNCTION
Dramatic reduction in platelet numbers is almost always caused by immune-mediated destruction, though certain tick-borne blood parasites could also be responsible (or could be the initiators of the immune response):
If an infectious agent such as one of these is responsible for the immune-mediated platelet destruction, obviously specific therapy against the infection is warranted in addition to therapy for the platelet destruction.
Very low platelet counts can also occur in response to the suppression of megakaryocytes within the bone marrow. This might be caused by:
Disseminated Intravascular Coagulation is a life-threatening disastrous uncoupling of normal blood clotting and clot dissolving functions in the body and one of its hallmark signs is a drop in platelet count (along with a constellation of other signs).
If platelet numbers are normal but it is obvious that platelet function is not, some other causes to look into might include:
THERAPY FOR IMMUNE MEDIATED PLATELET DESTRUCTION
Once a tentative diagnosis of immune-mediated platelet destruction has been made, the goal in therapy is to stop the phagocytes of the spleen from removing the antibody-coated platelets and cutting off antibody production. This, of course, means suppression of the immune system using whatever combination of medication seems to work best for the individual patient.
Prednisone or Dexamethasone
Azathioprine or Cyclophosphamide
(original graphic by marvistavet.com)
If medication simply does not work or the condition keeps recurring once medications are discontinued, the solution may be to simply remove the spleen. After all, this is where the phagocytes removing the platelets are primarily located. In humans, immune-mediated platelet destruction is generally treated with splenectomy first. Response in dogs has not been as predictably good thus in veterinary medicine it is generally one of the last therapies invoked.
Page last updated: 11/27/10