The immune system is an intricate network of specialized tissues, organs, cells, and chemicals.
Innate immunity is present at birth and provides the first barrier against microorganisms.
The skin, mucus secretions, and the acidity of the stomach are examples of innate immunity that act as a barriers to keep unwanted germs away from more vulnerable tissues. Adaptive immunity is the second barrier to infection. It is acquired later in life, for example after an immunization or successfully fighting off an infection. The adaptive immune system retains a memory of all the invaders it has faced. This is why people usually get the measles only once although they may be repeatedly exposed to the disease.