What gets into the walls of blood vessels and moves towards the accumulation of bacteria?

Leukocytes.
These are white blood cells that penetrate the walls of blood vessels and are sent to the area of the body where there is an accumulation of unwanted bacteria. If a person is sick, then an increase in the number of leukocytes is found in his blood. Because the main function of these blood vessels is to protect the body. Leukocytes protect the body from internal and external pathogenic substances, slowing down their growth and, ultimately, destroying them.