How does the serous membrane protects organs from infection?

The serous membrane  is a thin membrane that lines the internal body cavities and organs and is the first line of physical defense from microorganisms.

Also, serous membrane secretes cytokines and other molecules involved in membrane repair and inflammatory response. Some immune responses, like  coagulation and movement of white blood cells are under the control of serous membrane.

the sphincter muscles at the end of the digestive tract is the anus.