Human platelets. The mouse monoclonal antibody AK2 recognizes an extracellular epitope of CD42b (GPIb alpha), a 135-145 kDa membrane glycoprotein expressed on platelets and megakaryocytes. CD42b and CD42c (GPIb beta) are composed in a disulfide linked heterodimer (CD42b/c, 160 kDa), CD42b/c forms a noncovalent complex with CD42a and CD42d.