World Aquaculture 47 photo, the cell has been stained to highlight phenoloxidase. Sites of phenoloxidase activity are shown as red to pink. Once the parasite has been engulfed, phenoloxidase bombards toxic metabolites onto the parasite, killing it. QX outbreaks seem to occur when this phenoloxidasebased defense of oysters is compromised. A range of common environmental factors, such as low salinity, starvation or pollution can stress oysters, causing a decrease in phenoloxidase activity (Butt et al. 2006). This allows the parasite to break free of phenoloxidase’s normally effective control and cause QX disease. In addition to the killing systems that operate inside blood cells, invertebrates have a range of effector mechanisms that can be deployed outside cells. This is important because invaders can escape phagocytosis, often because they are simply too big to be ingested by a single blood cell. This is particularly true of large parasites, such as worms. When parasites are too large to ingest, they are walled off from the rest of the body by the process of encapsulation. Large numbers of blood cells surround the pathogen forming a tight seal, or capsule. A photo of this process is shown in Figure 3. In this image, a number of oyster blood cells hemocytes, revealed under UV light by their bright internal granules, are surrounding a fungal hypha to form a capsule. This process of encapsulation isolates parasites within infected animals, preventing them from proliferating and spreading infection. Defensive enzymes and other toxic molecules within the bright granules of the hemocytes are secreted into the capsule to kill the enclosed parasite. Phenoloxidase is one of the mechanisms used during encapsulation to kill parasites. Melanin produced by phenoloxidase is used to make the wall of the capsule rigid and prevent the parasite from escaping. It also helps to kill the parasites inside the capsule. Other pathogens escape phagocytosis not because of their large size, but because they have developed ways of evading defense cells, or even worse, hiding inside them without being killed. To counter this, the blood cells of most animals can secrete small antimicrobial proteins that have evolved to kill bacteria, as well as some fungi and protozoans. These proteins often work by “punching” holes in the surface of their targets. Most extracellular killing molecules are only secreted by blood cells when the presence of an infection is detected. In this way, potentially harmful killing systems are held in check until they are needed. Retaining killing systems within cells until they are required also helps to focus immune responses accurately at the site of infection. Infection usually causes inflammation, a response that increases the numbers of blood cells entering the site of an infection where the cells are stimulated to switch on their killing systems. Pattern Recognition As we said before, the effector (or killing) systems that can be brought to bear against pathogens by invertebrates are just as common and just as effective as those of vertebrates. Until recently it had been thought that the major difference between vertebrates and invertebrates was the way in which their immune systems are activated. In vertebrates, hypervariable antibodies are used to detect infection and switch on effector processes, such as phagocytosis. In contrast, invertebrates had been thought to rely on very different types of detection molecules that use a totally different method to identify infectious agents. This novel type of detection regime has been termed pattern recognition. Instead of using hypervariability to detect exact molecular structures found on the surface of individual microbes, pattern recognition detects structures that are common to large groups of different pathogens. The targets of pattern recognition are molecules, such as lipopolysaccharides (LPS), which are key building blocks that make up the surface of many different types of bacteria. By targeting such broadly distributed molecules, a single pattern recognition molecule, LPS-binding protein, can be used to detect a range of different bacteria. Other Fig. 4. Differences between specific immunorecognition and pattern recognition.
RkJQdWJsaXNoZXIy MjExNDY=