english lessons

Mixed Prepositions Exercise 20

Complete the text below by choosing the correct prepositions, then press "Check" to check your answers.
An immune system is a collection of mechanisms within an organism that protects infection identifying and killing pathogens. It detects pathogens ranging viruses parasitic worms and distinguishes them the organism's normal cells and tissues. Detection is complicated as pathogens adapt and evolve new ways to successfully infect the host organism. To meet this challenge, several mechanisms have evolved that recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess enzyme systems that protect viral infections. Other basic immune mechanisms evolved ancient eukaryotes and remain their modern descendants, such as plants, fish, reptiles, and insects. These mechanisms include antimicrobial peptides called defensins, pattern recognition receptors, and the complement system. More sophisticated mechanisms, however, developed relatively recently, with the evolution of vertebrates. The immune systems of vertebrates such as humans consist many types of proteins, cells, organs, and tissues, which interact an elaborate and dynamic network. As part of this more complex immune response, the vertebrate system adapts time to recognize particular pathogens more efficiently. The adaptation process creates immunological memories and allows even more effective protection future encounters these pathogens. This process of acquired immunity is the basis of vaccination.

Source: http://en.wikipedia.org/wiki/Immune_system
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