15.1 lists some of the ways in which arthropods affect the skin. Only a few can
be discussed here.
The skin changes are partly a result of the injection of pharmacologically active substances, and partly of sensitization to injected antigens.
A wheal may appear within a few minutes, to be followed
by a firm itchy persistent papule, often with a central haemorrhagic punctum.
Bullous reactions are common on the legs of children. The diagnosis is usually
obvious; when it is not, the term papular urticaria is sometimes used.