Antibody polypeptides that specifically bind human CD40L are provided. The antibody polypeptides do not activate platelets. The antibody polypeptides are useful in the treatment of diseases involving CD40L activation, such as graft-related diseases and autoimmune diseases. The antibody polypeptides may be domain antibodies (dAbs) comprising a single VH or VK domain. The half-life of the antibody polypeptides may be increased by modifying the antibody polypeptides to be dual specific reagents that can also bind human serum albumin (HSA) or another antigen.本發明提供特異性結合人類CD40L之抗體多肽。該等抗體多肽不活化血小板。該等抗體多肽適用於治療涉及CD40L活化之疾病,諸如移植相關疾病及自體免疫疾病。該等抗體多肽可為包含單一VH或VK域之域抗體(dAbs)。可由將該等抗體多肽修飾成亦可結合人類血清白蛋白(HSA)或另一抗原之雙特異性試劑來延長該等抗體多肽之半衰期。