Defence against infectious disease

• A pathogen is a disease-causing organism. 
• Wounds like cuts to skin cause openings which pathogens can potentially use to enter the body. Blood clots at wound site to prevent blood loss & entry of pathogens. Platelets (small cell fragments) along w damaged tissue release clotting factors in response to a wound. Clotting factors cause a series of rxns which end w fibrin (a protein) fibres forming a mesh across the wound site. The fibrin fibres capture blood cells & platelets forming a clot. In the presence of air the clot dries to form a scab that shields the healing tissues underneath. 
• Chemotaxis (movement in response to chemicals) attracts phagocytes to the area of invasion as response to: 

  • proteins produced by the pathogen
  • phospholipids released by damaged cells
• The phagocyte attaches to the pathogen’s cell surface proteins & then engulfs it. The fluid nature of the plasma membrane allows this to happen. A phagosome forms. Lysosomes – vesicles of digestive enzymes – deposit the enzymes into the phagosome. The digestive enzymes break down the pathogen & the waste products are expelled from the cell by exocytosis.