In fact more than 75% of our immune traits were found to be based on genetics.

Multiple high-quality studies and reviews indicate that both genetics and environment influence human immune traits, but the relative contributions vary by specific immune parameters. For example, a large twin study found that innate and adaptive immune traits are differentially affected by genetics and environment, with some traits more genetically influenced and others more environmentally shaped.nature.comnature.comInnate and adaptive immune traits are differentially affected by genetic and...

Several sources emphasize that environmental exposures such as infections, microbiome, and lifestyle factors often have a stronger impact on immune variation than genetics alone. Moreover, recent research highlights complex gene-environment interactions rather than a simple genetic predominance. While genetics certainly contribute to baseline immune function and susceptibility to certain diseases, the idea that over 75% of immune traits are genetically determined is not supported by the bulk of current evidence.med.stanford.edumed.stanford.eduEnvironment, not genes, plays starring role in human immune variation, study...science.orgscience.orgEnvironment, more than genetics, shapes immune system+3 more

Instead, the immune system's diversity and responsiveness result from a dynamic balance between inherited genetic factors and environmental influences throughout life.