A blood analyzer comprising: a sample preparing part configured to prepare a measurement sample by mixing a blood sample, a fluorescent dye for staining nucleic acid, and a hemolytic agent to hemolyze red blood cells; a light source configured to irradiate light on the measurement sample; a light receiving part configured to receive fluorescent light, forward scattered light, and side scattered light given off by the measurement sample irradiated by light, and output fluorescent light signals corresponding to the intensity of the fluorescent light, forward scattered light signals corresponding to the intensity of the forward scattered light, and side scattered light signals corresponding to the intensity of the side scattered light; and characterized by: a processing part configured to: discriminate and count reactive B lymphocytes based on the fluorescent light signals and/or forward scattered light signals, wherein the fluorescence light signals and/or the forward scattered light signals obtained from reactive B lymphocytes have stronger intensities than fluorescence light signals and/or forward scattered light signals obtained from normal lymphocytes and reactive T lymphocytes. a sample preparing part configured to prepare a measurement sample by mixing a blood sample, a fluorescent dye for staining nucleic acid, and a hemolytic agent to hemolyze red blood cells; a light source configured to irradiate light on the measurement sample; a light receiving part configured to receive fluorescent light, forward scattered light, and side scattered light given off by the measurement sample irradiated by light, and output fluorescent light signals corresponding to the intensity of the fluorescent light, forward scattered light signals corresponding to the intensity of the forward scattered light, and side scattered light signals corresponding to the intensity of the side scattered light; and characterized by: a processing part configured to: discriminate and count reactive B lymphocytes based on the fluorescent light signals and/or forward scattered light signals, wherein the fluorescence light signals and/or the forward scattered light signals obtained from reactive B lymphocytes have stronger intensities than fluorescence light signals and/or forward scattered light signals obtained from normal lymphocytes and reactive T lymphocytes. a processing part configured to: discriminate and count reactive B lymphocytes based on the fluorescent light signals and/or forward scattered light signals, wherein the fluorescence light signals and/or the forward scattered light signals obtained from reactive B lymphocytes have stronger intensities than fluorescence light signals and/or forward scattered light signals obtained from normal lymphocytes and reactive T lymphocytes. discriminate and count reactive B lymphocytes based on the fluorescent light signals and/or forward scattered light signals, wherein the fluorescence light signals and/or the forward scattered light signals obtained from reactive B lymphocytes have stronger intensities than fluorescence light signals and/or forward scattered light signals obtained from normal lymphocytes and reactive T lymphocytes.