Non-specific, or innate, immunity is the immune system's general-purpose, always-ready defence. It does not need to recognise a specific pathogen in advance, and it is traditionally described as acting in two lines, built on a broader set of four kinds of barrier.
The first line of defence consists of external barriers that try to stop a pathogen from ever entering the body at all — the skin, mucus lining the respiratory and digestive tracts (which traps particles before they can penetrate deeper), and secretions such as saliva and tears, which contain the enzyme lysozyme that breaks down bacterial cell walls.
The second line of defence comes into play once a pathogen has actually breached the first line: phagocytic cells that engulf the invader, natural killer (NK) cells that destroy infected or abnormal cells, the inflammatory response (localised redness, heat, swelling and pain as blood vessels dilate and immune cells are recruited to the site), and fever (a body-wide rise in temperature that makes conditions less favourable for many pathogens and speeds up immune cell activity).
Looked at more broadly, these defences fall into four types of barrier: physical barriers (the skin and the mucous membranes lining internal tracts); physiological barriers (body secretions such as the acidic pH of the stomach, and antimicrobial substances in saliva and tears such as lysozyme); cellular barriers (phagocytic white blood cells such as neutrophils and macrophages, and NK cells, which engulf or destroy pathogens directly); and cytokine barriers (virus-infected cells secrete signalling proteins called interferons, which warn and protect neighbouring, still-uninfected cells from the same virus).
A newborn baby's own immune system is still immature at birth, so it depends for a while on ready-made protection passed on by the mother. Colostrum, the thick, antibody-rich first milk produced in the days just after birth, is especially important because it is loaded with antibodies (mainly IgA), which coat the lining of the infant's gut and give it immediate, passive protection against infections until its own immune system is mature enough to take over.