When it triggers an abnormal or vigorous immune responce, treating the antigen that would normally cause no threat as something harmful.
<span>All living things are made up of cells, and depending on how complex the organism is, it may have one cell, like a bacterium, or trillions of cells, like humans. The question is, how does a fertilized human embryo with one cell develop into an adult with 5 trillion cells? The answer is in a process referred to as cell division. A single fertilized human egg cell will divide to produce two cells. The two cells have the same genetic information as each other and the parent cell. The cells will continue to divide until trillions of cells are produced. This is how a fertilized egg grows into a baby, and then an adult. This division of cells is referred to as mitosis.</span>
Cells are constantly dividing and replacing old or damaged cells.
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I got fossils as an answer, though I'm not 100% sure so I would check that.
Explanation:
