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Abstract We herein report a 79‐year‐old woman who underwent surgery had recurred non‐small cell lung cancer and developed irAEs following ICI treatment. During ICI treatment, we conducted monthly measurements of the serum antibody levels in this patient, including those which were both tumor‐ (anti‐p53 antibody) and nonspecific (immunoglobulins). Anti‐p53 antibodies and IgM had not increased during ICI treatment, but the serum levels of IgG and IgA had gradually increased before the occurrence of irAEs. These results suggest that monitoring serum immunoglobulin levels might enable the early detection of ICI‐induced immune responses in patients with lung cancer.