The cells of an organism can have attributes that are: 1. unique to each individual cell 2. unique to the organism, but identical among its cells (i.e. cells

The cells of an organism can have attributes that are: 1. unique to each individual cell 2. unique to the organism, but identical among its cells (i.e. cells descending from the same egg cell) 3. identical among all members of the species, or even across species Any immune system will necessarily evolve to use the attributes from group #2. If it were to use #1, the organism dies. If it were to use #3, then rogue cells from other compatible organisms can infiltrate and steal the organism’s resources, which is evolutionarily disadvantageous (unless it’s a beneficial relationship, c.f. gut microbiome).