[M95D]

Now we do understand why allergies appear, or at least we have theories that are being tested with studies such as this one. It's all about chance, very literally.

As lymphocytes are formed, they randomly rearrange their T-cell receptor / immunoglobulin genes, creating a random antigen specificity for each cell. [1]

Then, they get selectively killed if they react to self-antigens. [2]

Those that survive, if they ever meet their specific antigen, will selectively multiply [3] and do random mutations again [4].

  [1] https://en.wikipedia.org/wiki/V(D)J_recombination
  [2] https://en.wikipedia.org/wiki/Clonal_deletion
  [3] https://en.wikipedia.org/wiki/Clonal_selection
  [4] https://en.wikipedia.org/wiki/Somatic_hypermutation

The current theory is that allergies appear if: (1) some random lymphocyte rearrangement created affinity for that allergen and (2) the allergen was not "known to be safe" by the selection mechanisms of the body and that lymphocyte was allowed to survive.

[themafia]

> Then, they get selectively killed if they react to self-antigens. [2]

There's also the ones that react weakly to self antigens and are kept around to put a check on everything else.

https://en.wikipedia.org/wiki/Regulatory_T_cell

The thymus is just insane.

[tialaramex]

Well, Mother Nature doesn't have to make any sense, so that's an advantage to her. This does make our nature - to try to understand - extremely difficult but makes no difference to her either way.