The clone TS1/22 recognizes a 170-180 kDa alpha chain of a type I transmembrane glycoprotein also known as CD11a or LFA-1a. CD11a non-covalently associates with integrin β2 (CD18) to form LFA-1. CD11a is abundantly expressed on all leukocytes, including B and T lymphocytes, monocytes, macrophages, neutrophils, basophils and eosinophils but is not expressed by the non-hematopoietic tissues and platelets. Up-regulation of CD11a on activated lymphocytes is required to induce the immune response. Absence or reduced CD11a expression on tumor cells helps to escape from regular immune surveillance and may be a reason for in vivo tumor generation.