Agglutination, a molecular defence that involves production of molecules (agglutinins) that binds pathogens together by forming cross-links between short-chain carbohydrates on their surface. This immobilizes them near entry site, not allowing them to move about the host’s body, which makes it harder for them to acquire nutrients by diffusion. All animals have agglutinins, which is something they have in common with plants.
Wound closure is another type of molecular defence that prevents loss of body fluids and entrance of more pathogens. Examples of this are phyla like Annelida, Arthropoda and Echinodermata that contain specialized cells that congregate in wounds producing coagulants that cause body fluids to gel and block wounds.
In vertebrates, the main cascade reaction of innate immunity is complement cascade which has a final molecule called membrane attack complex that makes pores in pathogens, allowing access for other defensive mechanisms. In invertebrates, the cascade reaction is the PpO cascade that converts proplenoloxidase (PpO) to plenoloxidase, enzyme that destroys microbial pathogens.
Animals have evolved specialized defensive cells, called leukocytes in addition to their defensive molecules. Leukocytes, usually referred to as white blood cells, are the source of most defensive molecules. Leukocytes are motile and can ´burrow` between cells, penetrating deep into the hosts tissues. This is why not only they are found in the bloodstream, but also outside it, elsewhere in the body, mainly were the tissues are in contact with the external environment. They congregate in areas of infection or parasite infestation. In vertebrates, leukocytes are found in membrane-bond structures and also circulate in the lymphatic system.
There are various types of leukocytes: phagocytes capable of phagocytosis (ingest and kill single celled pathogens). Some pathogens are to large to be engulfed so another defence mechanism comes into play called cell-mediated cytotoxicity that requires direct cell-to-cell contact between leucocytes and pathogens. Leukocytes are also evolved in regulating and directing immune responses by synthesising and releasing cytokines, collective term for short-range signalling involved in coordinate and promoting immune defences. (Gillman, 2001)
Figure 1: Phagocytosis – A phagocyte engulfing a ed blood cell from another species. (taken and from p162 Fig5.6c)
Now we go onto plant defences, in particular chemical defences against herbivores where we can consider the group of nitrogen-containing compounds collectively referred to as alkaloids. When alkaloids are present at high concentration, they affect the growth and development of insects, altering blood pressure and respiratory rate in vertebrates and affecting neurotransmitters.
The plant apparency theory stated that the different defences between plant species depend on their apparency. Apparent plants were more easily found by herbivores and pathogens. There are two types of innate defence: quantative and qualitative. Quantative defences include chemical defences (tannis) that give tissues a bitter taste and resins, physical defence such as seed coat resistance, thorns, and thick cuticles where the quantity of defence is important. Qualitative defences are primarily chemical such as cyanogenesis and alkaloid defences, they are active in small amounts and inexpensive to synthesize. Apparent species (easier to find) produce primarily quantative defences (more) and unapparent species (Less easily found) produce primarily qualitative defences (low cost). (Gillman, 2001)
Plants, similar to animals also have induced defences where they respond to various types of stimulus: physical damage by switching on or amplifying existing defence systems. Studies of tobacco cells show that the plant responds with a cascade reaction detectable after 3-4 hours and at maximum force after 20-28 hours. The cascade defence reactions of plants and animals both involve induced defence, the first stage being tissue damage, after a series of stages the product is around the entrance site (damaged tissue). Until here everything very much the same. The big difference is the overall time reaction in animals it may be ten seconds where as in the plants it could be hours.
To conclude I can say that both animals and plants have molecular and cellular defences against pathogens, they have very similar defences that mainly differ in the time of reaction. The time of reaction of the animals is about ten seconds and plants are hours. This could have to do with the animals being more complex than the plants. The animals defence system is much more effective than the plants but not unbeatable.
Total words: 834
References:
- Gillman, M. (2001) Defence, in Generating diversity, M. Gillman, The Open University, Milton Keynes, p 153;
- Gillman, M. (2001) Defence, in Generating diversity, M. Gillman, The Open University, Milton Keynes, pp156-163;
- Gillman, M. (2001) Defence, in Generating diversity, M. Gillman, The Open University, Milton Keynes, pp 168-171;
- Gillman, M. (2001) Defence, in Generating diversity, M. Gillman, The Open University, Milton Keynes, pp 173-174;