This is what we do every day:
and everybody poos.
Our bodies' organs need just the right amount of water, temperature, energy intake and waste management to operate efficiently. This is called homeostasis; maintaining the same state despite all the changes about you and in you.
If some of those changes injure you, such as trauma or infection, it is the job of the immune system to help fight off an invading pathogen or germ, neutralize a toxic substance, and help coordinate the healing process after the harm has occurred. There are several components of the immune system, designed to protect your body against typical dangers, be tolerant of harmless substances and organisms, "good bacteria", for example. If a child or adult lacks one of these components, either an immune cell population or chemical, that individual may be at risk for severe or recurrent infections and/or hypersensitivity disorders, such as "allergies" or autoimmune disorders.
One out of two of us will develop a chronic disorder, whether a nuisance of a condition, such as urticaria (hives) or rhinitis (sometimes called 'hayfever'), or more worrisome disorders, such as recurrent sinus infections or asthma. To see if immune dysfunction is contributing to fatigue, rashes, headache disorders, or gastrointestinal distress, consider an evaluation by an allergy/immunology specialist.