A key strategy in keeping the body free from infection is to prevent the entry of harmful organisms in the first place. Barrier, or passive immunity, acts as a first line of defence against pathogens, providing protection via the physical and chemical barriers presented by the various surfaces of the body. These include both external surfaces such as the skin and mucus-lined internal surfaces like the airways and the gut. Each body surface forms an initial physical barrier to infection, and this is then supplemented by a variety of secreted substances that exhibit antimicrobial properties such as enzymes, which break down bacteria. Additional vital mechanisms function to expel or flush out microbes from the body like coughing, sweating and urination. If barrier immunity is breached, for instance by a skin wound, and pathogens enter the body, the innate immune system then becomes actively involved. Key to this is the activation of an inflammatory response and the deployment of immune cells. Tissue damage results from inflammation, which helps to prevent microbes from spreading. The capillary walls in the affected area become more penetrable, enabling immune cells to access the infected tissue. Damaged cells release chemicals that attract immune cells once they have migrated from the bloodstream.