McKenna's Pharmacology for Nursing, 2e

C H A P T E R 1 5  Introduction to the immune response and inflammation 229

myelocytes: leucocyte-producing cells in the bone marrow that can develop into neutrophils, basophils, eosinophils, monocytes or macrophages non-self cells: cells that are foreign; not identified as part of the organism itself phagocytes: neutrophils that are able to engulf and digest foreign material phagocytosis: the process of engulfing and digesting foreign pyrogens pyrogen: fever-causing substance rubor: redness, one of the four cardinal signs of inflammation; caused by activation of the inflammatory response T cells: lymphocytes programmed in the thymus gland to recognise self cells; may be effector T cells, helper T cells, or suppressor T cells tumour: swelling, one of the four cardinal signs of inflammation; caused by activation of the inflammatory response

T he body has many defence systems in place to keep it intact and to protect it from external stressors. These stressors can include bacteria, viruses, other foreign pathogens or non-self cells , trauma and exposure to extremes of environmental conditions. The same defence systems that protect the body also help to repair it after cellular trauma or damage. Understand­ ing the basic mechanisms involved in these defence systems is important in order to be able to explain the actions of the drugs that affect the immune system and inflammation. BODY DEFENCES The human body can mount two types of immune defences. The innate or natural (or non-specific) immunity and the specific or acquired immunity. The innate or natural immunity defences are factors that are present in the body prior to exposure to a particular infectious organism. The specific or acquired immune defences occur only after exposure to substances called antigens. The body’s defences include barrier defences, cellular defences, the inflammatory response and the immune response. Each of these defences plays a major role in maintaining homeostasis and preventing disease. Barrier defences Certain anatomical barriers exist to prevent the entry of foreign pathogens and to serve as important lines of defence in protecting the body. These barriers include the skin and mucous membranes, gastric acid and the major histocompatibility complex. Skin The skin is the first line of defence. The skin acts as a physical barrier to protect the internal tissues and organs of the body. Glands in the skin secrete chemi­ cals that destroy or repel many pathogens. The top layer of the skin sheds daily, which makes it difficult for any pathogen to colonise on the skin. In addition, normal bacterial flora of the skin help to destroy many disease- causing pathogens.

Mucous membranes Mucous membranes line the areas of the body that are exposed to external influences but do not have the benefit of skin protection. These body areas include the respiratory tract, which is exposed to air; the gastro­ intestinal (GI) tract, which is exposed to anything ingested by mouth; and the genitourinary (GU) tract, which is exposed to many pathogens from the perineal and rectal area. Like the skin, the mucous membrane acts as a physical barrier to invasion. It also secretes a sticky mucous capable of trapping invaders and inacti­ vating them for later destruction and removal by the body. In the conducting airways of the respiratory tract, the mucous membrane is lined with tiny, hair-like processes called cilia . The cilia sweep any captured pathogens or foreign materials upwards towards the mouth, where they will be swallowed. The cilia can also move the captured material to an area causing irritation, which leads to removal by coughing or sneezing. In the GI tract, the mucous membrane serves as a protective coating, preventing erosion of GI cells by the acidic environment of the stomach, the digestive enzymes of the small intestine and the waste products that accu­ mulate in the large intestine. The mucous membrane also secretes mucous that serves as a lubricant through­ out the GI tract to facilitate movement of the food bolus and of waste products. The mucous membrane acts as a thick barrier to prevent foreign pathogens from pene­ trating the GI tract and entering the body. In the GU tract, the mucous membrane provides direct protection against injury and trauma and traps any pathogens in the area for destruction by the body. Gastric acid The stomach secretes acid in response to many stimuli. The acidity of the stomach not only aids digestion, but also destroys many would-be pathogens that are either ingested or swallowed after removal from the respira­ tory tract. Major histocompatibility complex The body’s last barrier of defence is the ability to distin­ guish between self cells and foreign cells. All of the cells