Final answer:
The body's first line of defense includes physical and chemical barriers such as the skin, tears, mucus, stomach acid, urine flow, and friendly bacteria. These barriers prevent most pathogens from entering the body. If the barriers are breached, inflammation and cellular responses occur.
Step-by-step explanation:
Your body's first line of defense includes the skin and other barriers, such as tears, mucus, cilia, stomach acid, urine flow, and friendly bacteria, that keep most pathogens out of your body. The skin acts as a mechanical barrier, physically blocking pathogens from entering the body. Chemical barriers, such as enzymes in sweat, saliva, and semen, kill pathogens on body surfaces. Biological barriers, like harmless bacteria, prevent pathogenic bacteria from colonizing the body.
If pathogens breach these protective barriers, inflammation occurs, creating a physical barrier against the spread of infection and repairing tissue damage. Inflammation is triggered by chemicals like cytokines and histamines, and it causes swelling, redness, and warmth. The complement system, consisting of proteins, disrupts the cell membrane of pathogens. Cellular responses involve leukocytes, such as neutrophils, macrophages, and dendritic cells, that phagocytize pathogens, and basophils and mast cells that trigger inflammation. Natural killer cells destroy cancerous or virus-infected cells, and eosinophils kill parasites.