What are some of the non-specific, first-line immune defences in humans?

When we think of immune defences, our minds may immediately jump to white blood cells such as macrophages or T lymphocytes, however there are actually many mechanisms by which our bodies fight infection by preventing pathogens entering in the first place. The simplest example is skin - it not only forms a tough outer layer thanks to keratin produced by the epidermis, but also makes for an antimicrobial environment due to lactic acid and fatty acids secreted by sebaceous glands. Antibacterial substances such as the lysozyme, a hydrolase enzyme, can be found in tears, and saliva. The flow of urine, on the other hand, acts as a flush in the urinary tract.Parts of the body in contact with the environment such as the airway are lined in mucous membranes. Within these, goblet cells produce mucus to which bacteria may stick. Then, ciliated cells waft this mucus towards the back of the throat where it may be swallowed. This brings us onto another point - the stomach. The low pH, due to hydrochloric acid produced by parietal cells, kills the bacteria by denaturing. Lastly, further down the GI tract, there is a role for commensal bacteria colonising the gut - they compete for nutrients and may even produce their own compounds to keep pathogenic bacteria (e.g. C. difficile) from overgrowing.