Old age is the result of ageing or senescence which may be defined as the progressive deterioration in structure and function of the body cells, tissues, organs and organ systems of the organism with their advancing age. Here, the body's immune system loses its ability to effectively deal with the antigens. This reduces the resistance as well as adaptation of the body. Cessation of mitosis is not as such a genetically programmed event. During ageing, there is a decrease in multiplication of cells, though it differs in various tissues.