Immunology: Animal Models

Most immunologic activities decline with age, but some show an increase, and a few show no significant change. In principle, two types of cellular changes could alter immune functions: changes in the number of immune cells (quantitative change), and changes in the functional efficiency of immune cells (qualitative change).

Quantitative change. A modest loss of circulating lymphocytes (15 percent) has been observed in aging humans, but in mice the total number of immune cells does not change appreciably with age. Thus, cell loss does not appear to contribute significantly to changes in immune function with age.

Qualitative change. Metabolic, morphologic, and genetic studies present impressive evidence for age-related qualitative changes in immune cells. An important role in the regulation of immune reaction belongs to monocytederived cells (MDCs). These cells originate from monocytes, which leave the blood and differentiate into various types of tissue macrophages. Lymphocytes are the mediators of immunity, but their function is under the control of MDCs. The MDC system not only activates lymphocytes, it also makes lymphocytes tolerant or unreactive to self antigens, lymphocytes to self-antigens, thereby minimizing autoimmune reactions.

Studies in mice have shown that, with age, MDCs have a reduced capacity to stimulate proliferation and differentiation of lymphocytes. These results suggest an age-related shift in the regulatory activities of MDCs in the absence of a change in their numbers.