McKenna's Pharmacology for Nursing, 2e

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P A R T 3  Drugs acting on the immune system

( phagocytosis ). When the body is injured or invaded by a pathogen, neutrophils are rapidly produced and move to the site of the insult to attack the foreign substance. Because neutrophils are able to engulf and digest foreign material, they are called phagocytes . Phagocytes are able to identify non-self cells by use of the MHC, and they can engulf these cells or mark them for destruction by cytotoxic T lymphocytes. Basophils Basophils are myelocytic leucocytes that are not capable of phagocytosis. They contain chemical substances or mediators that are important for initiating and main­ taining an immune or inflammatory response. These substances include histamine, heparin and other chemi­ cals used in the inflammatory response. Basophils that are fixed and do not circulate are called mast cells . They are found in the respiratory and GI tracts and in the skin. They release many of the chemical mediators of the inflammatory and immune responses when they are stimulated by local irritation. Eosinophils Eosinophils are circulating myelocytic leucocytes whose exact function is not understood. They are often found at the site of allergic reactions and may be responsible for removing the proteins and active components of the immune reaction from the site of an allergic response. Monocytes/Macrophages Monocytes or mononuclear phagocytes are also called macrophages . They are mature leucocytes that are capable of phagocytising an antigen. Macrophages help to remove foreign material from the body, including pathogens, debris from dead cells and necrotic tissue from injury sites, so that the body can heal. They can

and tissues of each person are marked for identification as part of that individual’s genetic code. No two people have exactly the same code. In humans, the genetic iden­ tification code is carried on a chromosome and is called the major histocompatibility complex (MHC). The MHC produces several proteins called histocompatibil­ ity antigens, or human leucocyte antigens (HLAs). These antigens (proteins) are located on the cell membrane and allow the body to recognise cells as being self cells. Cells that do not have these proteins are identified as foreign and are targeted for destruction by the body. Cellular defences Any foreign pathogen that manages to get past the barrier defences will encounter the human inflamma­ tory and immune systems, or mononuclear phagocyte system (MPS). Previously called the reticuloendothelial system, the MPS is composed primarily of leucocytes, lymphocytes, lymphoid tissues and numerous chemical mediators. Stem cells in the bone marrow produce two types of white blood cells or leucocytes : the lymphocytes and the myelocytes. The lymphocytes are the key components of the immune system and consist of T cells, B cells and natural killer cells (see later discussion of the immune response). The myelocytes develop into a number of different cell types that are important in both the basic inflammatory response and the immune response. Mye­ locytes include neutrophils, basophils, eosinophils and monocytes, or macrophages (Figure 15.1). Neutrophils Neutrophils are polymorphonuclear leucocytes that are capable of moving outside of the blood stream (dia­ pedesis) and engulfing and digesting foreign material

Natural killer cells

Lymphocytic cells

B cells

Plasma cells

Antibodies

Cytotoxic T cells

Helper T cells

T cells

Suppressor T cells

Bone marrow stem cells

Neutrophils

Eosinophils

Myelocytic cells

Basophils

FIGURE 15.1  Types of white blood cells, or leucocytes, produced by the body.

Monocytes/macrophages