McKenna's Pharmacology for Nursing, 2e

74

P A R T 2  Chemotherapeutic agents

called histocompatibility antigens or human leucocyte antigens (HLAs), which the body uses to identify a cell as a self-cell (i.e. a cell belonging to that individual). The body’s immune system recognises these proteins and acts to protect self-cells and to destroy non–self-cells. When an organ is transplanted from one person to another, a great effort is made to match as many histocompatibility antigens as possible to reduce the chance that the “new” body will reject the transplanted organ. Histocompatibility antigens can be changed in several ways: by cell injury, with viral invasion of a cell, with age, and so on. If the markers are altered, the body’s immune system reacts to the change and can ignore it, allowing neoplasms to grow and develop. The immune system may also attack the cell, leading to many of the problems associated with autoimmune disorders and chronic inflammatory conditions. Channels Channels or pores within the cell membrane are made by proteins in the cell wall that allow the passage of small substances into or out of the cell. Specific channels have been identified for sodium, potassium, calcium, chloride, bicarbonate and water; other channels may also exist. Some drugs are designed to affect certain channels specifically. For example, calcium-channel blockers prevent the movement of calcium into a cell through calcium channels.

lipids—phospholipids, glycolipids and cholesterol; the bipolar arrangement of the lipids monitors substances passing into and out of the cell. The phospholipids, which are bipolar in nature, line up with their polar regions pointing towards the interior or exterior of the cell and their non-polar region lying within the cell membrane. The polar regions mix well with water, and the non- polar region repels water. These properties allow the membrane to act as a barrier to regulate what can enter the cell (see Figure 7.2). The freely-moving nature of the membrane allows it to adjust to the changing shape of the cell so that areas of the membrane can move together to repair itself if it should become torn or injured. Some of the outward-facing phospholipids have a sugar group attached to them; these are called glycolipids. Choles- terol is found in large quantities in the membrane, and it works to keep the phospholipids in place and the cell membrane stable. Receptor sites Embedded in the cell membrane are a series of periph- eral proteins with several functions. As discussed in Chapter 2, one type of protein located on the cell membrane is known as a receptor site. This protein reacts with specific chemicals outside the cell to stimu- late a reaction within the cell. For example, the receptor site for insulin reacts with the hormone insulin to cause activation of adenosine triphosphate (ATP) within the cell. This reaction alters the cell’s permeability to glucose. Receptor sites are very important in the func- tioning of neurons, muscle cells, endocrine glands and other cell types, and they play a very important role in clinical pharmacology. Identifying markers Other surface proteins are surface antigens, or genetic­ ally determined identifying markers. These proteins are

KEY POINTS

KEY POINTS

■■ The cell is the basic structure of all living organisms. ■■ The cell membrane features specific receptor sites that allow interaction with various chemicals, histocompatibility proteins that allow for self-identification, and channels or pores that allow for the passage of substances into and out of the cell.

Receptor sites

Carbohydrate

Extracellular fluid

Polar regions, mix well with water

Non-polar region, repels water

Proteins

Lipid bilayer

Histocompatibility antigens

Cholesterol

Protein channel

Cytoplasm

FIGURE 7.2  Structure of the lipid bilayer of the cell membrane.

Phospholipids