MODEL RELEASED. Scientists screening blood at the North London Blood Transfusion Centre. The man on left is performing a Fuji-Rebio test for antibodies to HIV-1 (Human Immunodeficiency Virus type 1), a causative agent of AIDS. Human serum is diluted then placed in both a test and a control well. Gelatine particles & HIV antigens are added to the test well but only gelatine particles to the control well. Any HIV-antibodies present in serum agglutinate with antigens in the test well & provoke the appearance of a lattice in the bottom of the well. A small button" of particles would be the appearance of a normal control sample."