Biology lesson 6:
  Immune system:
     – Dr Sayan Bhattacharyya 
  The immune system gives us immunity or protection from different diseases.   Immunity can be natural or innate, and acquired or adaptive   Natural immunity is present since birth, and comprises components like skin, gastric acid, Tear, Saliva, Defensin, Mucus, Neutrophils etc. 
  Skin is the largest organ in the body. Skin and mucosa have Langhans giant cells that can capture foreign antigens and present it to the immune system. Skin also forms a barrier and prevents entry of microorganisms inside human body. 
 Neutrophils engulf the microbes by phagocytosis. Then the phagosome fuses with Lysosone and Lysosomal enzymes kill the microbes .
  A subset of WBCs are the NK cells or Natural Killer cells.  They play a role in hypersensitivity and also target and kill tumour cells and virus infected cells. 
 Tear and Saliva contain Lysozyme that kill all bacteria except Staphylococcus aureus. 
 Defensins are low molecular weight proteins that damage cell wall of bacteria.  They are produced by Neutrophils and other cells, and are found in gut.
 In next class we will study in more details the immune system.
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