In previous chapters, you learned about osmotic and diffusion gradients that aid in maintaining water and ion balance and in developing resting potentials in cells, among other important biological features. how does the immune system of vertebrates exploit a chemical gradient?

In vertebrates, the immune system discharges chemical signals known as chemokines that produce a chemical gradient, which is utilized by the leukocytes in migrating towards the site of infection.  

Chemokines belong to the class of small cytokines, or signaling proteins discharged by the cells. Their name is obtained from their tendency to stimulate directed chemotaxis in the adjacent responsive cells. They are chemotactic cytokines.  

the second type of galaxy is called a barred spiral, which has a thing called a bar straddling the nucleus