‘Identity’ is not a metaphor either.  A shoe is not a metaphor for itself or for another shoe.  It is a shoe. They are both shoes, that is their identity. ((I am not suggesting that object of identity cannot change, or that proper names are neither lost nor created.  Not to be a metaphor is not the same as having some variable extension.))

They both have structural features in common: they both have a sole that protects the foot, they both have a method of attachment, and both designs take into account the movements of the foot.  It is the relation of one shoe to a different shoe that I claim for the relation of allergy to phobia. That is they possess or share in vital part the same structure or essence.

Some features of phobia

Some features of allergy

1. The object of phobia is generally harmless, at least in relation to the anxiety it produces.   For Freud’s ‘Little Hans’ the object of his phobia were horses.  Horses can be dangerous but in Vienna some 100 years ago they were ordinarily used for transport, generally without hazard. 1. The object of allergy is generally harmless, at least in relation to the anxiety it produces.   For hay fever sufferers the object is pollen.  If someone has hay fever then pollen can cause the same physiological effects as phobia, even unto death.  Otherwise pollen is harmless.  The immune system of the hay fever sufferer has attached a great importance to something otherwise trivial.
2.  The onset of phobia is variable but perhaps typically around age three.  A child’s phobia is regarded as a plastic structure, something transitional and is generally made light of by those experienced in rearing children: an adult with a similar phobia would probably be referred to a professional for help.  Children with phobias typically ‘grow out of them’ or seem to. 2.  Onset is often early but is variable.  Children with allergies typically ‘grow out of them’ or seem to.  Late onset can mask an underlying autoimmune disease, which it guards against.
3.  Phobia is theorised by Freud and Lacan in the case of Little Hans as being caused by too little repression, that is, by something absent.  Little Hans gave an importance to horses that he was unable to attribute to his own father.  In his family it was his mother who was dominant, his father’s position was opaque: horses took a place of authority, where the father of Little Hans was lacking.  Horses took the place of the absent father.  What exactly was missing for Little Hans?  An agent able to separate him from his mother.  That is a sufficiently identifiable non mother. 3.  Allergy is currently theorised as being caused by too little repression, that is, by something absent.  Allergy is very rare in the third world and very common in the first.  Why?  The current paradigm in immunology is the ‘cleanliness hypothesis’ that argues that the developing immune system of a young child requires exposure to something foreign, ideally bacteria.  Should there be insufficient viruses or bacteria that the child’s immune system comes into contact with then it typically reaches for something harmless that it has already made contact with such as pollen.  The immune system then treats the harmless allergen, the pollen with an exaggerated importance: a potentially fatal immune response is made.  In the third world childrens’ immune systems typically have plenty of contact with viruses and bacteria, so they do not have to compensate by becoming allergic.Note the absent element here is sufficient exposure to viruses and bacteria.  Viruses and bacteria are non mother.  That is they are collectively an agent that marks the child as different from the mother: if in general a child had an immune response to his mother’s body then it is likely that the child would have been spontaneously or ‘naturally’ aborted.