Communicable diseases

• host - the organism the pathogen infects
• Types of pathogens

  • virus
  • bacteria
  • fungi
  • protista
• Virus

  • HIV/AIDS , influenza, tobacco mosaic virus
  • non living
  • cause host cell to manufacture copies of the virsus causing it to bust
• Bacteria

  • Tuberculosis , ring rot ( potatoes/tomatoes ) , bacterial meningitis
  • release toxins inhibiting host cells functions
• Fungi

  • black sigatoka (bananna) , athletes foot , ring worm
  • uni cellular / multicellular
  • release enzymes and digest material around them,
  • products of digestion are absorbed back into fungal cells- causing damage to host cells/tissues
• Protista

  • malaria , potato / tomato late blight
• Direct transmission of pathogens in plants

  • pathogens in soil infect plant by entering roots (easier if damaged by weather, replanting or burrowing)
  • fungi spores produced sexually/asexually are carried in the wind - airborne transmission
  • pathogens infect vascular tissue, carry to and from soil to infect other plants can enter seeds and be distributed so offspring are infected
• Indirect transmission of pathogens in plants

  • spores/bacteria become attached to a burrowing insect which acts as a vector while attacking plants
• Diseases grow and reproduce more rapidly in warm, moist climates
• Indirect transmission between animals can occur via a vector
• Vector - organism used by pathogen to gain entry to the primary host
• Direct transmission of pathogens between animals - social factors

  • overcrowding
  • poor ventilation
  • poor health / diet
  • homelessness
  • contact with people who have migrated from areas where a disease is more common
  • drinking contaminated water
  • contact with infected faeces or animals
• Physical plant defences:

  • cellulose wall / bark - contains chemical defences
  • lignin cell wall - waterproof / indigestible thick barrier
  • waxy cuticles - prevents water collecting on cell surfaces
  • stomatal closure - close when pathogens are detected
  • callose
  • tylose
• Role of Callose in physical plant defences -

  • large polysaccharide deposited in cell walls reinforced with lignin
  • blocks sieve tube end plates in phloem preventing spread of infected area
  • blocks plasmodesmata between infected and nearby cells
• Role of Tylose in physical plant defences -

  • fills xylem vessel preventing spread of pathogens
  • contains high conc. of toxins
• Chemical plant defences are produced after plant develops an infection requiring lots of energy
• Chemical plant defences -

  • Tannins - bitter taste from enzymes
  • Hydrolytic enzymes - break down the chitin in the cell wall (Chitinases )
  • insect repellent (pine resin / lemongrass)
  • insecticides (caffeine - neurotoxin )
  • antibacterial compounds - disrupt the cell wall of bacteria
  • Anti-oomycetes - break down the cell walls on fungi
• Primary non-specific defences:

  > digestive system

  > respiratory system

  > skin

  > inflammation

  > ear wax
• Primary defence in digestive system

  • hydraulic acid
  • mucus lining of stomach
  • lysosome enzymes in saliva / tears
• Primary defence in respiratory system

  • hair /mucus in nostrils and trachea
  • ciliated epithelium and goblet cells
  • expulsive reflexes (coughing and sneezing)
• Primary defence in skin

  • skin made of keratin -strong and waterproof
  • vector organisms can break this skin
  • infection triggers clotting
• Clotting process in skin

  • endothelium is damaged
  • platelets exposed to outside proteins and are activated, triggering blood clotting
  • plug is formed over damaged area
  • releases clotting factor chemical thromboplastin
  • thromboplastin + Ca2+ in blood act on protein prothrombin -> enzyme thrombin
  • thrombin acts on soluble fibrogen forming insoluble fibrin
  • forms mesh, trapping RBCs ->clot
  • activated platelets release chemical histamine
  • causing smooth muscle cells in blood vessel wall to contract, narrowing blood vessel reducing blood flow to area
• Blood clots form scabs on skin surfaces

  • protects underlying tissue from pathogens whilst healing occurs
  • skin cells under scab divide and repair tissue damage
• Inflammation

  damaged tissue activates mast cells, releasing histamine

  • causes nearby blood vessels to vasodilate- increasing blood supply to infected area
  • causes a temperature increase, reducing pathogens ability to reproduce
  • blood vessel walls become more permeable, more blood plasma and WBCs leave blood forming tissue fluid causing nearby cells to swell
• Damaged tissues release Mast cells which release cytokines:

  • attract phagocytes to damaged tissue for phagocytosis
  • trigger the hypothalamus, raising body temperature, reducing pathogens ability to reproduce - fever
• First line of secondary defence is phagocytosis - specialised cells engulf pathogens and destroy them:

  • phagocytes
  • neutrophils
  • macrophages
• Antigen - glycoprotein on plasma membrane of pathogen that triggers and immune response
• Opsonins - bind to antigen on surface of pathogen allowing phagocytes to bind
• Phagocytes -specialised cells in blood/tissue fluid that engulf and digest pathogens
• neutrophils (phagocyte)- WBC that engulf foreign matter and traps it in a phagosome which fuse with lysosomes to digest
• Macrophages are antigen presenting cells and travel as monocytes
• Macrophages -

  • engulf pathogen via endocytosis
  • antigens are saved and moved to special protein complex on macrophage surface
  • becomes an antigen-presenting cell
  • exposes antigen to immune system for recognition
  • protein complex prevents the cell from being identified as foreign
• Cytokines stimulate the differentiation of macrophages, B and T lymphocytes
• Phagocytosis process by neutrophils

  • opsonins bind to antigens
  • phagocytes recognise and bind to opsonin by chemotaxis
  • phagocyte engulfs pathogen by endocytosis, forming a phagosome
  • lysosomes fuse with phagocytic vacuole releasing hydrolytic enzymes into it
  • digestion and killing of pathogen
• Phagocytes specialisations:

  • receptors on plasma membrane that bind to opsonin or antigen
  • lobed nucleus that allows that allows cell to squeeze through narrow gaps
  • well developed cytoskeleton - cells changes shape to engulf pathogen and move lysosomes/ vacuoles in the cell
  • many lysosomes
  • many mitochondria to release energy from glucose
  • many ribosomes to synthesise enzymes needed
• Blood smear

  A) Platelet

  B) RBC

  C) neutrophil

  D) neutrophil

  E) monocyte

  F) lymphocyte

  
• B lymphocytes form and mature in the bone marrow
• T lymphocytes form and mature in the thymus gland
• T cells have complementary receptors on their surface that bind to specific antigens
• Differentiation of T cells:

  • T helper cells
  • T killer cells
  • T memory cells
  • T regulator cells