First Line of Defence

The first line of defence against infection are the surface barriers that prevent the entry of pathogenic substances

In plants these barriers are of particular importance, as plants lack the cellular defences that animals possess

Surface Barriers in Animals

The main surface barriers in animals are the skin and mucous membranes


  • Protects external structures (outer body areas)
  • A dry, thick and tough region made of predominantly dead surface cells
  • Contains biochemical defence agents (sebaceous glands secrete chemicals which inhibit the growth of some bacteria)
  • The skin also releases acidic secretions to lower pH and prevent bacteria from growing

Mucous membranes

  • Protect internal structures (externally accessable cavities and tubes, such as trachea, vagina and urethra)
  • A thin region containing living surface cells that release fluids to wash away pathogens (mucus, tears, saliva, etc.)
  • Contains biochemical defence agents (secretions contain lysozyme, which can destroy cell walls and cause cell lysis)
  • Mucous membranes may be ciliated to aid in the removal of pathogens (along with physical actions such as coughing or sneezing)

Summary of Surface Barriers in Humans

Surface Barriers in Plants

Plants have no distinct immune system of the kind found in humans and most animals

Instead they have a number of structural and biochemical characteristics that provide protection against pathogens

Mechanical Barriers

  • The waxy cuticle and outer epidermal cells provide a physical barrier for plants in the same way that skin does
  • Many plants employ certain structures, including thorns, spines and prickles, to deter animals (and hence avoid potential vector-based pathogens)
  • When pathogens gain access to a plant via stomata (a common point of entry), layers of thickened cells (called cork) may form and create protuberances (called galls) which limit the spread of the pathogen within the plant
  • Some plants may even have mechanisms that result in the rapid death of tissue (apoptosis) that are under attack, thereby depriving invading organisms of organic matter

Chemical Barriers

  • Some plants resist disease by producing chemicals that have antimicrobial properties
    • Saponins are a group of inactive compounds stored in vacuoles that may damage the cell membrane of pathogens when released
  • Defensins are proteins which block the growth of pathogens and are typically released from germinating seeds
  • Plants may additionally secrete a range of other anti-bacterial or anti-fungal agents, including resins and tannins
  • Some trees will produce oils that repel certain insect pests (and associated vector-borne diseases)
  • Other plants may secrete gum around infected areas, essentially sealing the area off