6.7—Pathogen entry routes and barriers

Pathogens can enter through broken skin, food and drink, the respiratory tract or mucosal surfaces. Skin, clotting, gut flora, stomach acid and lysozyme reduce entry or survival before a specific immune response is needed.

Each barrier uses a different mechanism: intact skin blocks access, a clot seals damage, resident microbes compete, acid harms swallowed pathogens and lysozyme damages bacterial walls.

A cut bypasses the skin barrier, but clotting limits further entry; a swallowed pathogen must also survive stomach acid and compete with gut flora before reaching intestinal tissue.

A barrier lowers risk rather than guaranteeing protection. Do not confuse lysozyme’s bacterial-wall action with a general antiviral response.