The third section of the samll intestine. Fat-soluble vitamins, fats, bile salts, and cholesterol are primarily absorbed in the ileum. 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. Unfortunately some bugs such as the viruses that cause the common cold disguise themselves and they must be fought off time and again by the immune system. The importance of the mind and of emotional stress in immune function is now recognized in the relatively new field of psychoneuroimmunology.