While the physical presentation can manifest in many different forms, dog acne is essentially due to a build-up, clog, or bacteria, oil, and dead skin cells inside a hair follicle. The build-up of extraneous matter causes the pore to clog up and forms a sore on the hair follicle’s surface. The oil that clogs up pores is known as sebum. Your dog’s skin naturally generates sebum as an extra layer of protection between your dog’s skin and coat. Many factors can result in an overproduction of sebum, such as inflammation in the follicle, hormonal changes, and bacteria. When bacteria is stuck inside the follicle and cannot be expelled quickly, your dog’s body’s natural defense response is to send white blood cells to protect the infected area. The overflow of white blood cells is what causes the nasty-looking pus that can come out of pimples.