Disclosed herein are immunoglobulins, such as antibodies, and antigen binding portions thereof, that specifically bind complexes of GARP-TGFβ1, LTBP1-TGFβ1, LTBP3-TGFβ1, and/or LRRC33-TGFβ1. The application also provides methods of use of these immunoglobulins for, for example, inhibiting TGFβ1 activity, and treating subjects suffering from TGFβ1-related disorders, such as cancer and fibrosis.