Many microbiologists separate the human body's lines of defenses into primary, secondary, and tertiary lines of defense.

Primary defenses against pathogens are based on the fact that intact body surfaces pose a great barrier against penetration of microbes. Intact skin and mucous membranes form an effective barrier against microbes. Sebaceous glands associated with hair follicles produce fatty acids and lactic acid which lower the pH and inhibit the growth of pathogenic microbes. Even thouth the hair follicle is an appealing habitat for bacteria and fungi, good skin hygiene typically removes the transcient microbes that inhabit the follicles.

Microbes that enter the body through the nose or mouth are trapped by nasal hairs or ciliary action and removed in the mucous through coughing or swallowing. Many potential pathogenic microbes that are swallowed are destroyed by somach acids and exposed to the abundant microflora (microbes) in the intestine. The beneficial microbes in the intestine keep reproducing during the 24 hour passage of food through the intestine. Since feces weight may be up to 50% dead bacteria, it is important that the normal flora continue to be replaced. Loss of normal intestinal microbes may result in the opportunistic growth of Staphylococcus, Proteus, and Candida (yeast).

In addition to intact skin, mucous, stomach acids, and intestinal microbes; the kidney and surface of the eye are washed and cleaned with secretions containing lysozyme. Lysozyme is an enzyme that digests bacterial cell walls. Also blood contains blood proteins called beta-lysins that disrupt the cell membranes of bacteria. The human body has numerous first line or primary body defenses. Tissues of the body have the ability to prevent the establishment by certain pathoges-- for instance Clostridium tetani is killed by stomach acids and therefore is dangerous as a pathogen only in a puncture wound. Salmonella does not cause wound infections but could possibly cause intestinal infections.

Finally the low pH of the vagina produced by acids released by Lactobacillus, the acidic pH of the urine, bile, and the flow or urine reduce the possible infection by parasites and other microbes.