<P> The two main categories of white blood cells are granulocytes and agranulocytes . </P> <P> Neutrophils (a type of granulocyte) are the most abundant of all the WBCs . </P> <P> Platelets, or thrombocytes or yellow blood cells, are very small, irregularly shaped clear cell fragments (i.e. cells that do not have a nucleus containing DNA), 2--3 μm in diameter, which derive from fragmentation of precursor megakaryocytes . The average lifespan of a platelet is normally just 5 to 9 days . Platelets are a natural source of growth factors . They circulate in the blood of mammals and are involved in hemostasis, leading to the formation of blood clots . Platelets release thread - like fibers to form these clots . </P> <P> If the number of platelets is too low, excessive bleeding can occur . However, if the number of platelets is too high, blood clots can form thrombosis, which may obstruct blood vessels and result in such events as a stroke, myocardial infarction, pulmonary embolism--or blockage of blood vessels to other parts of the body, such as the extremities of the arms or legs . An abnormality or disease of the platelets is called a thrombocytopathy, which can be either a low number of platelets (thrombocytopenia), a decrease in function of platelets (thrombasthenia), or an increase in the number of platelets (thrombocytosis). There are disorders that reduce the number of platelets, such as heparin - induced thrombocytopenia (HIT) or thrombotic thrombocytopenic purpura (TTP), that typically cause thromboses, or clots, instead of bleeding . </P>

Which is the smallest blood cell in human body