Abstract: The present invention provides a method for reversible labeling an antigen A of a biological specimen comprising said antigen A with a monovalent antigen binding molecule K comprising a binding site B for specifically binding said antigen A, the method comprising the steps a) contacting said biological specimen comprising said antigen A with said antigen binding molecule K, thereby generating a complex of A/K, b) removing the monovalent antigen binding molecule K that is not bound in said complex of A/K, wherein the equilibrium dissociation constant (KD) for the binding of said antigen A and said monovalent antigen binding molecule K has a value of equal or greater than 0.5E-08 M and wherein the dissociation rate constant (k(off)) of said binding has a value of equal or greater than 1E-03 sec-1. In some embodiments of the invention a multimerization component F for multimerization of monovalent antigen binding molecules (F/K complex) is included in the method, wherein said F/K complex can be disrupted.

Abstract: The present invention provides a humanized antibody or fragment thereof specific for the antigen CD3, wherein said antibody or fragment thereof comprises a heavy chain variable domain comprising a CDR1 region of SEQ ID NO: 1, a CDR2 region of SEQ ID NO:2, and a CDR3 region of SEQ ID NO:3, and a light chain variable domain comprising a CDR1 region of SEQ ID NO:4, a CDR2 region of SEQ ID NO:5, and a CDR3 region of SEQ ID NO:6. In addition, the present invention provides a method for polyclonal stimulation of T cells comprising contacting a population of T cells with said anti-CD3 antibody, optionally with an anti-CD28 antibody, wherein said anti-CD3 antibody and optionally said anti-CD28 antibody are linked to particles. Further said anti-CD3 antibody can be used for reversible tagging of cells.