One aspect of the present disclosure provides antibodies that can act as agonists of PD-1, thereby modulating immune responses regulated by PD-1. Another aspect of the disclosure provides compositions comprising PD-1 specific antibodies and their use in methods of down regulating the immune response. These methods can be practiced on any subject, including humans or animals. Anti-PD-1 antibodies disclosed herein may be used, in another aspect of the invention, to detect PD-1 or its fragments in a biological sample. The amount of PD-1 detected may be correlated with the expression level of PD-1, and associated with the activation status of immune cells (e.g., activated T cells, B cells, and/or monocytes) in the subject.