What is the the theme for the book Enrique's Journey

An obvious theme that is presented in this text is that of immigration, primarily of the illegal nature.

Enrique sets off alone from Tegucigalpa, with little more than a slip of paper bearing his mother’s North Carolina telephone number. Without money, he will make the dangerous and illegal trek up the length of Mexico the only way he can–clinging to the sides and tops of freight trains.

With gritty determination and a deep longing to be by his mother’s side, Enrique travels through hostile, unknown worlds. Each step of the way through Mexico, he and other migrants, many of them children, are hunted like animals.