Final answer:
The body's first line of defense consists of three different types of barriers: mechanical, chemical, and biological. These barriers physically block pathogens, kill them on body surfaces, or prevent them from colonizing the body. Inflammation and immune cells also play a role in defending against infection.
Step-by-step explanation:
The body's first line of defense consists of three different types of barriers that keep most pathogens out of body tissues. The types of barriers are mechanical, chemical, and biological barriers. Mechanical barriers which include the skin, mucous membranes, and fluids such as tears and urine-physically block pathogens from entering the body.
Chemical barriers such as enzymes in sweat, saliva, and semen-kill pathogens on body surfaces.
Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.
If pathogens breach the protective barriers, inflammation occurs. This creates a physical barrier against the spread of infection and repairs tissue damage. Inflammation is triggered by chemicals such as cytokines and histamines, and it causes swelling, redness, and warmth. The complement system is a complex biochemical mechanism that helps antibodies kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the cell membrane of pathogens and bursting of the cells. Cellular responses of the innate immune system involve various types of leukocytes (white blood cells). For example, neutrophils, macrophages, and dendritic cells phagocytize pathogens. Basophils and mast cells release chemicals that trigger inflammation. Natural killer cells destroy cancerous or virus-infected cells, and eosinophils kill parasites.