Immune System And The Elderly

The Immune System

The immune system is the body’s defensive network and protect s individuals a gainst billions of microorganisms in the environment. The system is complex, being made up of many types of cells and proteins that perform different functions in fighting foreign substances. Foreign substances are called antigens. The response of the immune system to the introduction of foreign substances is called the immune response. The immune response takes place within the peripheral lymphoid tissues. These include the lymph nodes, spleen and un-encapsulated tissues lining the respiratory, and genitor-urinary tracts. The lymph nodes filter off, and if necessary, respond to foreign materials entering body tissues. The spleen monitors the blood while un-encapsulated lymphoid tissues defend the mucosal surfaces. Communication between these tissues and the rest of the body is carried out by re- circulating lymphocytes which pass from the blood into the lymph nodes, spleen and other tissues; and back to the blood by the major lymphatic channels. The immune response can be innate or acquired.

Innate or natural immunity involves mechanisms that pre-exist the invasion of foreign substances. These include physical barriers like the skin and mucosal surfaces, chemical substances (e.g. complement proteins and lysozymes) that counteract microorganisms and other foreign particles; and specialized cells (phagocytes) that engulf and digest foreign particles. The mechanisms of innate immunity are non-specific, i.e., they treat different foreign substances alike. Complement proteins meet foreign invaders early on invasion. They flow in the blood and quickly reach the

site of invasion. They require activation to perform their functions which include the triggering of inflammation, attraction of phagocyte cells to the area of invasion and coating of intruders for easy access to them by phagocytes. They can also kill foreign bodies by cytolysis. Phagocytes are immune cells with char acteris tics of locating and engulfing of microorganisms and dead or injured body cells for destruction. Phagocytes include the granulocytes, macrophages and dendritic cells. Macrophages also play a key role in alerting the rest of the immune system to the presence of invaders. Like macrophages, dendritic cells help with the activation of the rest of the immune system. They also filter body fluids to rid them of foreign organisms and particles.

Acquired immunity is generated strictly in response to a given foreign agent. The immunity improves in Cytotoxic T cells specialize in attacking virus-infected cells and cancer cells. B cells are produced, and mature, in the bone marrow. They form plasma cells when activated by contact with antigen and help from helper T cells. Plasma cells produce antibodies which are capable of destroying the antigen type responsible for their production. Antibodies link foreign antigens that they coat to phagocytes and also to complement proteins. They also neutralize toxins and incapacitate viruses from infecting new cells.

The Immune System in the Elderly

Immune functions decline with age, i.e., immune competence declines as people grow old. Ageing affects the immune system at multiple levels including reduced B cell and T cell production in the bone marrow and

thymus. Furthermore, mature lymphocytes in peripheral lymphoid tissues are characterized by diminished function. As a result, elderly people do not respond to immune challenges as actively as do the young. For instance, individuals of 70 years of age and older people have increased vulnerability to influenza infection1.

One major change that occurs during ageing is the process termed thymic involution. The thymus naturally atrophies as humans age. Although T cells are produced throughout the life span, the progressive decay of the thymus causes significant decrease in the number of T cells and alters their sub-type distribution. The progressive loss of the thymus has profound effects on the immune system of the aged. Ageing affects the functions of T cells in many ways. This includes depletion of the population of naïve T cells since fewer T cells are produced as the thymus progressively deteriorates. Consequently, the aged immune system cannot respond as well as a young one to new antigens. The cell surface of all T cells undergo significant changes during ageing. When a T cell binds to an antigen, the stimulus must be communicated to the interior of the T cell for activation of the cell to take place. The process of transmitting the antigen-binding signal across the cell membrane into the cell is called signal transduction. Signal transduction is a cascade of chemical reactions involving many molecules. Aged T cells do not display the surface CD28 antigen, a molecule that is critical for signal transduction and T cell activation2. Such T cells remain quiescent and do not respond to foreign agents. A decrease in calcium is another defect of T cell activation in the elderly. Calcium is crucial for many biochemical reactions, including signal transduction. Calcium deficiency results in failure of stimulation of required enzymes that need it to function properly2. As T cells age, their capacity to produce or respond to the cytokine IL-2 is lost, thus greatly impairing T cell function.

Reduced function of T cells in old people affects B cell function as well since helper T cells are required for the activation of B lymphocytes. Activated B cells develop into plasma cells which produce antibodies. The presence of auto-antibodies (i.e., antibodies that react to self components, against normal principles), is more prevalent in the aged, even in healthy aged without auto- immune disease. Although the exact role of auto- antibodies in ageing is not fully elucidated, they may cause slowly progressive tissue damage which contributes to physical ageing.