The BCG vaccine primarily prevents infection by the tuberculosis bacillus, so it can prevent pulmonary tuberculosis. The protective effect of the BCG vaccine is mainly achieved through inducing a cell-mediated immune response. After vaccination, a primary infection is formed, which is processed by macrophages that transmit the antigen information to immune-active cells, leading to the differentiation and proliferation of T cells and the formation of sensitized lymphocytes. When the body is re-infected with the tuberculosis bacillus, the macrophages and sensitized lymphocytes are activated, causing a specific immune response in the body, thereby resisting the infection by the tuberculosis bacillus. Therefore, the BCG vaccine mainly prevents pulmonary tuberculosis.