Τόμος 10 (1992) – Τεύχος 1 – Άρθρο 1 – Επιθεώρηση Κλινικής Φαρμακολογίας και Φαρμακοκινητικής-Ελληνική Έκδοση – Volume 10 (1992) – Issue 1 – Article 1 – Epitheorese Klinikes Farmakologias και Farmakokinetikes-Greek Edition

10: 5-29 (1992)

PHARMAKON-Press: Epitheorese Klin. Farmakol. Farmakokinet.

10: 5-29 (1992)

immunology, specific immune responses, antigens, antibodies, humoral immunity, cell-mediated immunity, lymphocytes, B-lymphocytes, plasma cells, immunoglobulins, null cells, natural killer cells, natural cytotoxic cells, T-lymphocy­tes, T-helper cells, T-suppressor cells, cytotoxic T-cells, lymphokines

Immunology is a relatively young branch of medical science; it influences our understanding, as well as diagnosis and therapy of many different illnesses. Immunolo­gy deals with the specific immune responses, that is with the defense mechanisms which operate only when a person has been previous­ly exposed to a particular substance. Substan­ces capable of stimulating immune responses are known as antigens, which can stimulate the production of antibodies. There are two kinds of specific immune mechanisms: humoral immunity, which involves the production of antibodies by B-cells, and cell–mediated im­munity, in which special T-lymphocytes react directly with foreign material.

Lymphocytes have the pivotal role in the immune system. They bear the major respon­sibility for the actions of the immune system. They cannot phagocytize, but instead demon­strate other special properties.

– Lymphocytes carry specific surface receptors which allow each cell to react with one indivi­dual antigen. This is the basis of immunologi­cal specificity: (a) Lymphocytes divide after stimulation by antigen (and the presence of growth factors) to form cell clones of identical specificity. They are long-lived (months or years) and can retain information about anti­gens for a long time. This is the basis of im­munological memory. (b) Lymphocytes can cir­culate between the bloodstream, the lymphatic system and tissues. This ensures that specific memory, acquired as a consequence of a local immune reaction, is distributed throughout the whole body.

When lymphocytes leave the bone marrow, they consist of two large groups. One group moves to the thymus and is there processed (acquisition of new surface antigens) and matu­rated (under the influence of the hormone thy-rosin) to become effective or immunologically competent. These cells later leave the thymus as T-lymphocytes. The other group of cells tra­vels directly to the lymphoid tissues where they become S–lymphocytes.

B-lymphocytes, when exposed to foreign antigens, they synthesize RNA and differentiate to plasma cells, which produce and secrete immunοglobulines (antibodies) into the body fluids (“Humor”). They thus contribute to what is called humoral immunity. The antibodies re­act with circulating antigens, but are unable to enter living cells.

T-lymphocytes, in contrast, react directly with their targets, i.e. with body cells infected by viruses, or transformed into tumor cells. T-cells therefore contribute to what is called cel­lular immunity.

Functionally distinct lymphocytes all look the same under the light microscope. However, they carry different markers (glycoproteins) on their surfaces, which can be identified with the help of monoclonal antibodies. In this way it is possible, not only to distinguish B- and T-cells, but also to discriminate further subpo­pulations of these. Lymphocyte-like cells which possess neither T- nor B-cell markers are re­ferred to as Null cells. Null cells comprise the Natural Killer (NK) cells and the Natural Cyto­toxic (NC) cells.

Each B-cell is programmed to produce a specific antibody. When a B-cell receives an antigenic stimulus, it divides to give rise to se­veral large plasma cells which produce and secrete immunoglobulins (antibodies). The sur­face of the B-cell is also enveloped in the antibodies, which allow the B-cells to recogni­ze the specific antigens. Most B-cells are under the regulatory control of the T-lymphocy­tes (T-H cells stimulate; T-S cells suppress).

T-cells have two important functions: they orchestrate the interplay of the different im­mune cells, and are also able to directly attack body cells which have been infected by viruses or are transformed into tumour cells. Functio­nally, T-cells can be divided into the following groups: (a) Τ-Helper cells: activate B-cells, stimulate other T-cells, NK cells and macro­phages. (b) T-Suppressor cells: inhibit comple­tely or in part B- and other T-cells. (c) Cytoto­xic T-cells (killer T-cells): locate and destroy virus-infected body cells. (d) Delayed hypersen­sitivity T-cells: attract and activate other cells, e.g. macrophages.

All T-cells which participate in cell-mediated immunity can become involved in the forma­tion of memory cells, and hence can lead to a secondary, later immune response of increased effectiveness. The effects of T-cells can be at­tributed largely to the secretion of lymphoki­nes, which include factors responsible for the attraction (chemotaxis) of macrophage cells and also lymphotoxin (which kills foreign cells) and interferon, which inactivates viruses.

Cellular immunity plays an important role in the body’s defence against viral, bacterial and fungal infections, in transplant rejection and in the defence against neoplastic growths. The presence of T-cell immunity to tuberculosis can be demonstrated with the Mantoux reaction (tuberculin reaction).

Overactivity of T-cell immunity causes one form of hypersensitivity reaction. T-lymphocyte function may be suppressed by steroid hormo­nes or by the drug azathioprine. Such drugs are used to prevent rejections of a grafted tis­sue or organ.

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