In the present invention, inventors report the characterization of BCLA (Brain Cyst Load-associated Antigen), a protein exclusively expressed during the bradyzoite stage of the parasite. In cysts directly purified from the brain of mice, the protein is distributed within and at the surface of the cyst. ELISA antibody capture using a combination of serologically reactive BCLA peptides and a recombinantly expressed c-terminal domain (rBCLA) constitutes an efficient serological marker of latent infection with a high sensitivity that is clearly and exclusively correlated with the presence of cysts in the brain of mice. Antibodies directed against BCLA antigen have been detected in human patients with enriched titers in patients qualified as seropositive to Sag1 or tachyzoite related antigens. Further correlation in humans between anti-BCLA IgG synthesis and cysts is brought by significantly stronger recorded titers in pathological panels strongly related to the presence of cyst.rnThus the invention relates to a new Toxoplasma gondii protein, hereafter referred as BCLA, a new serological marker whose expression is restricted to the latent form of Toxoplasmosis (bradyzoite/cyst). This specific protein and its antigenic fragments can be used to detect autoantibodies in the sera of patient for the diagnosis of the latent form of Toxoplasmosis. The invention also relates to derived antibodies, generated by BCLA immunisation that specifically binds this new proteinrn