Well first, your immune system has two main halves, innate and adaptive... the innate is the line of defense to fight off disease and bacteria, which is made up of anatomical barriers
genes is the answer
A gene mutation is a permanent alteration in the DNA sequence that makes up a gene, such that the sequence differs from what is found in most people. Mutations range in size; they can affect anywhere from a single DNA building block (base pair) to a large segment of a chromosome that includes multiple genes.
