In traditional medical classification there are 4 basic types of animal tissue: epithelial, connective, muscle, and nervous.
Epithelial tissue lines the surfaces and cavities of body structures, including forming the structure of glands. Epithelial cells secrete, absorb, protect, sensate, and transport other cells.
Epithelial surfaces are washed by tears, saliva, and urine. Many secreted body fluids contain bactericides: acid in gastric juice, lactoperoxidase in mother’s milk, spermine and zinc in semen, and lysozyme in tears, snot, and saliva.
Mucus secreted by membranes lining the inner surfaces of the body acts to block bacteria from adhering to epithelial cells. Sticky mucus traps pathogens, which are then removed by various means, including crawling (ciliary movement), sneezing, and coughing.
The stomach is a highly acidic environment: helpful for breaking down food and killing microbes that are eaten.
The body’s gut flora assist. The contented families of commensal bacteria don’t like competition for nutrients, so friendly microbes suppress the growth of potentially pathogenic bacteria and fungi.
For example, the vagina is protected by lactic acid, which is produced by commensal microbes which metabolize glycogen. The glycogen supply is secreted as microbial food by vaginal epithelium. When protective commensals are disrupted by antibiotics, the door is opened to opportunistic pathogens such as Candida albicans and Clostridium difficile.