This article summarizes some experimental and clinical data about immunologic determinants of etiopathogenesis of periodontitis. The main etiological factor of periodontitis is bacterial plaque deposited on the tooth surface. In the healthy oral cavity there is a state of balance between bacterial antigens and host immune system. The pathogenic bacteria interact with periodontal tissue, stimulating local immune reaction within the oral cavity. Chronic periodontitis is the most prevalent periodontal disease in adults. The immunologic inflammatory reaction, leading to periodontal tissue destruction, is initiated by Gram-negative bacteria. Oral mucosa is infiltrated by immune cells, which take part in cellular immune reactions and humoral response. The research directions in the field of immunologic aspects of periodontitis, pointed out in this paper, clearly indicate the complexity of the pathological processes, leading to destruction of periodontal tissue.