The following is the Cleveland ClinicThe immune system is composed of proteins and chemicals, as well as organs and white blood cell. Your immune system’s first defense against bacteria and fungi, parasites and toxins is the skin and mucous membranes. Skin produces oils and cells that kill infectious substances before they can get in. Mucus is produced by the mucous membranes and traps foreign invaders trying to enter the body. Then, the cilia — hair-like structures located throughout the body — move the invaders out of the body before they can infect healthy cells.
If pathogens make their way past the skin and mucous membranes they will be eliminated by several other systems. To catch intruders, the tonsils in your throat and the adenoids in the throat behind your nose work together. They then release chemicals that kill them.
The stomach and digestive tract function in the same way. Pathogens can be killed by chemicals when they reach the stomach or bowel. Good bacteria fights bad bacteria in the gut.
The core of your immune system is made up of bone marrow, lymph nodes and thymus. Infected cells can be infected by invaders that make their way into the body. An army of white cells from the body will respond to the infection.