Some scientists refer to immune senescence, a term that describes the progressive decline in function of the immune system with age. Some components lose function; others increase functions inappropriately. Various aspects of the immune system respond to aging in different ways. Immune senescence occurs gradually. The chronic diseases of aging can also aggravate the effects of immune senescence.

Natural immunity and aging
One component of natural immunity is the macrophage, the cell whose job it is to engulf and destroy invading cells. Weak T cells in older adults can slow the activation of macrophages.

Adaptive immunity and aging
The thymus gland is the organ in the chest that assists in the maturation of T cells. The thymus involutes or shrinks as we mature, and can be nonfunctional by the age of 60. How significant this is to immunity in old age is unclear. T cell immunity does drop off with age, but most of the T cells have passed through the thymus already by the time it has involuted.

Older adults produce fewer T helper cells, and the ones they do have are often less effective than they were in earlier life. Others show aberrant function.

Finally, it appears that many autoimmune diseases arise in older adults, though the incidence of new cases likely peaks in our 30's and 40's. The immune system's ability to recognize "self" diminishes as we age, and we can begin to produce autoimmune antibodies. This may be due to age-related changed that take place within cells, such as oxidative damage from free radicals and glycation, the inappropriate insertion of glucose (sugar molecules) into DNA and other cell components. Such small changes to our cells may cause our immune systems to fail to recognize them as our own, and lead to the production of autoantibodies. It is not clear whether these autoantibodies produce disease as often in older adults as they do in those who are younger.

Mucosal immunity and aging
In humans, the production of antibody by the cells located in mucosal tissues such as that found in the mouth and intestines, falls with age, reducing mucosal immunity.

Allergic response and aging
This is perhaps the only place in which there is good news for older adults. We tend to produce less IgE, the antibody associated with the allergic response to such items as pollen and animal danders. Thus, we might be fortunate enough to outgrow our allergies as we age.