Innate and non-specific resistance is that which all pigs are born with or, in the case of some forms of non-specific resistance, that which develops as the pigs grow regardless of what infections they are exposed to. They consist of:

Physical barriers - These are the external and internal body surfaces of the respiratory, alimentary and urogenital tracts, namely skin, horn and hair externally; and internally, the membranes that line the nose, sinuses, throat and the tracheal and bronchial air tubes, the mouth, stomach and intestines; the vagina, uterus, bladder and urinary tubes. Other internal barriers include joint capsules and the blood-brain barrier.

Chemical barriers - The mucous on the surface of mucous membranes and some other body fluids such as saliva and tears contain antibacterial and anti-viral substances such as lysozyme and interferon. The acid in the stomach also inactivates bacteria and viruses to some degree.

Complement system - This is a series of at least 20 proteins (enzymes) in the blood that attack foreign cellular material in a sequential cascading manner, the first one or two acting and stimulating the next to act and so on. "Foreign" means non-self, i.e. material that is not part of the pig's normal body. Although complement acts non-specifically its action may be enhanced by some specific antibodies.

You may wonder how compliment distinguishes between "self" and "non-self". It does so because the pig's own cells have a coating of special pig-protective protein that acts like Teflon in a non-stick saucepan. It specifically stops the pig's own complement from sticking to surfaces of its own cells to ensure they are not destroyed. If it can not stick, it can not destroy. Viruses, bacteria and parasites do not possess this special pig protein so they are not protected.

Phagocytosis - Certain defence cells, called phagocytes, can engulf foreign material such as an invading bacterium in an attempt to destroy it within the body's tissue or to carry it away (e.g. to the gut) in order to eliminate it from the body.

Phagocytes fall into two groups, polymorphs and monocytes.

The first group, the polymorphs, are part of the blood white cell population. They circulate in the blood but respond quickly when pathogenic infection occurs. They rapidly migrate out of the blood stream to attack the infection. Some virulent bacteria are covered with a slippery capsule which inhibits the polymorphs from engulfing them. A specific antibody (called an opsonin) is then required to stick to the capsule enabling the polymorph to engulf it. One type of polymorph (eosinophil) tends to attack parasites.

The second group of phagocytes; monocytes, also start initially as part of the blood white cell population. They circulate in the blood to start with but then migrate into the tissues and onto inner body surfaces to become local tissue macrophages or they wander through the tissues as wandering macrophages. These macrophages are usually capable of engulfing viruses and bacteria non-specifically, i.e. without the aid of specific antibodies.

Bacteria, viruses, parasites and toxins contain antigens. Antigens stimulate antibody production.

Macrophages engulf bacteria and viruses in order to destroy them or remove them form the body but they are not always successful. Some virulent bacteria and viruses can survive and multiply inside macrophages often destroying the macrophages. For example, the virus of PRRS (porcine reproductive and respiratory syndrome) multiplies in the macrophages of the lungs and destroys them thus compromising the lung immunity.

Virulent strains of the bacterium Streptococcus suis type 2 and some other bacteria that cause meningitis, behave like the Greek legend of the Trojan horse. To get through the blood-brain barrier, which is normally resistant to penetration by bacteria, they hide in migrating monocytes. The monocytes then migrate through the blood-brain barrier to become brain macrophages where the bacteria break out to cause meningitis.

Probiotic flora - The skin, mouth, stomach, intestines, vagina and prepuce have a complex mixture of non disease producing organisms, mostly bacteria, that have evolved over thousands of years to live in intimate relationship with each other. They are usually limited to one part of the host. Some of them are antagonistic to invading pathogenic bacteria and inhibit their growth. Examples in the gut are the inhibitory activities of bacteroides organisms against E. coli or lactobacillus against salmonella. This 'indigenous flora' is not inherited, i.e. piglets are not born with it. However it is related to the pig's individual characteristics because it differs from the indigenous flora of other species. It is always the same mixture in any individual pig throughout life. As the pigs grow this indigenous flora becomes increasingly complex.

Lactobacillus bacteria inhibit the multiplication of other pathogenic bacteria by secreting lactic acid. Lactic acid is available commercially to put in water or feed in order to reduce bacterial multiplication.

Various combinations of probiotic bacteria, (usually streptococci and/or lactobacilli) are also available commercially for feeding to newborn pigs or to restore the flora of pigs which have been lost through treatment with oral antibiotics. Experimentally probiotics have been shown to be effective but they are sometimes disappointing when applied on the farm.