As an individual reaches adulthood, hematopoiesis is restricted to selected bones in the . More is replaced with fat as individuals continue to age. Thus, older individuals may be more prone to developing , which is a decrease in the number of circulating erythrocytes. In addition, older bone marrow may be less able to meet any demands for an increased number of formed elements. The in the elderly may be less efficient and active, and they may in number. Certain types of also are more prevalent among the elderly, probably due to the immune system being less efficient.