1.1　Basics of the Human Immune System

The human immune is extremely complex.It has evolved over hundreds of millions of years to respond to invasion by the pathogenic microbes that regularly attempt to infect our bodies, and invasion by the microbes that tried to infect our genetic ancestors.There are similarities between the immune system of humans and those of the most primitive of vertebrates, going back five hundred million years on the evolutionary ladder.(https://www.daowen.com)

The immune system does not rely on one single mechanism to deter invaders, but instead uses many strategies, the most important of which are detailed below.The main division between the strategies is that between innate imm unity, which does not require previous exposure to the invading microbe, and acquired immunity, whereby the immune system “remembers” how to deal with a microbe that it has dealt with before.