Evolution and the Non-Vertebrate Immune System


Types of Immune Cells

  • Leukocytes:
    • Neutrophils - surrounds foreign body and digest it with enzymes
    • Eosinophils - engulfs and kills foreign bodies
    • Basophils - mediates hypersensitivity reactions
    • Monocytes - removes damaged tissues
    • Lymphocytes - group name for Natural Killer cells, T cells and B cells
    • Natural Killers - kills foreign bodies
  •  Mast cells - mediate inflammatory response
  • Dendritic cells - antigen presenting cells
  • Erythrocytes
  • Platelets
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Evolution of Defence Systems

Evolutionary arm race:

  • Always been attacked by other organisms
  • Co-evolution occurs when adaptive changes in one organism favour selection adaptations in the other organism
  • Mate choice can be influenced by physical signs of immune systems eg. brighter feathers = healthier
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General Defence Systems

Internal Defence Systems

  • Immunity
  • Defence against invading pathogens
  • Removal of aged cells and debris from damaged tissues
  • Identification and destruction of abnormal/mutant cells

External Defence Systems

  • Deals with predators and competitors
  • Anatomic eg. spines
  • Chemical eg. toxins
  • Behavioural eg. escape response
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Internal Immunity

Internal immunity is divided into the innate and acquired immune systems.

The Innate Immune System

  • Works rapidly upon exposure to any foreign body
  • Does not depend on prior exposure 
  • Carried out by all animals

The Acquired Immune System

  • Targets a specific foreign body following first exposure
  • Delayed response
  • Found in all jawed vertebrates - although recent research suggests that invertebrates may have some form of acquired immunity


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Innate Immune Systems in Invertebrates

  • Barrier tissues act as first line of defence
  • Second line of defence includes phagocytic cells, inflammation, encapsulation, antimicrobial peptides (eg. defensin) and opsonin proteins (tag and invader so can easily be recognised again)
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Acquired Immune Systems in Invertebrates

Most insects do not have acquired immunity, however they do have cellular and humoral responses.

Cellular Response

  • Haemopoietic tissues (found near heart - produces haemocytes)
  • Fat bodies (produces immune peptides - found in locusts)

Humoral Factors

  • Phenoloxidase - enzyme that helps polymerise molecules into melanin
  • Lysosymes allow cell lysis to occur
  • Antimicrobial proteins
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  • Produces melanotic nodules that limit pathogen spreading
  • Respond to massive bacterial infection or multicellular organisms
  • Similar to clotting in vertebrates
  • Melanin forms around wounds and pathogens
  • Hardening gel traps pathogens which are destoryed by humoral peptides
  • However, requires a lot of energy
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Why Do Mosquitoes Not Get Malaria?

  • Malaria parasite: Plasmodium - single celled protozoa that must incubate in the mosquito for 10-21 days
  • National Institute of Allergy and Infectious Disease:
    • 2 groups of mouse blood were infected with Plasmodium
    • 1 group became infected but other didn't - placed in a room too hot for Plasmodiumto grow
    • 7 days later both groups were reinfected with Plasmodium
    • The infected group was up to 10 times better at killing Plasmodium
  • Granulocytes (type of leukocytes):
    • The team found 3 times as many of these immune cells in the pre-infected group than they did in the unifected groups
    • When granulocytes detect a foreign body in a mosquito's blood, they can either kill in themselves or signal another cell to come and do the job
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