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• Lysozyme

  An enzyme found in the saliva, airways and tear fluid that destroys the cell walls of bacteria
• Mucus in the bronchi


  • Helps trap many of the germs we breathe in, so they can be moved out of the airways by the hair-like structures (the cilia)
• Stomach acid


  • Stops most of the germs that enter the body in the food we eat
• Harmless bacteria on our skin and many of the mucous membranes


  • Act as part of the immune system
• Reflexes that cause us to cough and sneeze


  • Help to free our airways of germs
• Cells of the Immune System


  • White blood cells in the human bloodstream
  • Lymphocytes
  • Monocytes or agranulocytes
  • Polymorphonuclear leukocytes (PMN) or granulocytes
• Antibodies


  Help the body to fight microbes or the toxins (poisons) they produce by recognising substances called antigens on the surface of the microbe, or in the chemicals they produce, which mark the microbe or toxin as being foreign, and then mark these antigens for destruction
• Complement system


  Made up of proteins whose actions complement the work done by antibodies
• The body's first line of defense against microbes


  • Mechanical barriers
  • Chemical barriers
  • Biological barriers
• Skin


  • The most important mechanical barrier, tough and very difficult for pathogens to penetrate
• Mucous membranes


  • Provide a mechanical barrier at body openings, secrete mucus to trap pathogens, have hair-like cilia to sweep mucus and pathogens toward body openings
• Sweat, mucus, tears, and saliva


  • Contain enzymes that kill pathogens
• Urine


  • Too acidic for many pathogens
• Semen


  • Contains zinc, which most pathogens cannot tolerate
• Stomach acid


  • Kills pathogens that enter the GI tract in food or water
• Harmless bacteria on skin and in GI tract


  • Use up food and space so harmful bacteria cannot grow
• The body's second line of defense against microbes


  • Skin: waterproof barrier that secretes oil with bacteria-killing properties
  • Lungs: mucous (phlegm), cilia wave
  • Digestive tract: mucous lining contains antibodies, stomach acid kills most microbes
  • Other defences: body fluids like skin oil, saliva and tears contain anti-bacterial enzymes, constant flushing of urinary tract and bowel
  • Fever: immune system response, rise in body temperature
• Memory cells


  Types of white blood cells (B and T-lymphocytes) that keep a record of every microbe the immune system has ever defeated, allowing it to recognize and destroy the microbe quickly if it enters the body again
• Catching a cold or flu from one virus does not give you immunity against the others