The various mechanisms that protect the pig from infectious agents can be considered in six groups:

• Complement system - This is a non specific protective mechanism that acts on any foreign cells or viruses that do not possess certain pig proteins on their surface. It consists of a number of chemicals found in the plasma which act together as a cascade to remove or destroy organisms.
• Chemical factors - These include non specific enzymes (such as lysozyme in saliva) and acids which may be found in mucus, saliva and gastric juices. These immobilise or kill pathogens.
• Mechanical factors - These include the skin, mucus, sweat, lining of the nose, mouth, oesophagus, intestine, colon, vagina, flow of urine and the passage of faeces.
• Macrophage cells - These are found throughout the body in tissues and in the blood stream where they are called monocytes. They engulf and digest bacteria. They also have an important role in controlling viral and fungal diseases. The cells are of two types called leucocytes and monocytes.
• Specific acquired immunity - This is of two types; that which is activated by cells and called cell mediated immunity and antibodies present in the blood called humoral immunity. Cell mediated immunity arises when T type lymphocytes come into contact with antigens and they are stimulated to produce antibodies. It takes 7-14 days for these to develop. Humoral immunity is produced from B lymphocytes which have met the antigen previously and their response is immediate. Some lymphocytes also kill other cells that contain antigens or they may act immediately against antigens.
• Immunoglobulins - Specific antibodies of which there are different types namely immunoglobulins, IgG, IgM and IgA. They are found in blood, in milk and particularly in colostrum. All internal surfaces of the body also contain them.

Certain infectious agents can suppress the immune system sufficiently to make the animal more susceptible to other infections. Examples are Mycoplasma hyopneumoniae, aujeszky's disease virus, pasteurella bacteria, swine influenza and porcine reproductive respiratory syndrome virus (PRRS) all of which cause pneumonia.

Terminology
Adjuvant - A substance added to an inactivated vaccine to make it more effective.
Antibodies - Complex large proteins (called gamma-globulins) which are produced by specialised cells in response to invading antigens. These stick specifically to the invading antigen neutralising it or triggering off a destructive reaction.
Antigen - Foreign invading substance (i.e. a substance which is not normally part of the pig's body), usually consisting of protein or part of a protein, which stimulates the body to produce antibodies. Antigens exist on the surfaces of bacteria, viruses and parasites.
Antiserum - Serum with high antibody levels against a specific infection. It has usually been produced experimentally in laboratory animals by injecting the infection into them.
Blood sample - Whole blood sample taken hygienically with a syringe into a bottle or by a pin prick through the skin absorbing the droplet of blood with blotting paper.
Commensal bacteria - Bacteria that live permanently in or on the body without causing disease.
Epithelium - Cellular membrane (e.g. mucous membranes) containing epithelial and other cells.
Hyperimmune antiserum - The same as antiserum above but emphasising its high titre.
Lymphocytes - Specialised defence cells in lymph nodes, other lymphatic tissue and the blood which produce antibodies or take part in cellular immunity.
Mucous membranes - Cellular membranes (e.g. those lining the gut) which secrete a sticky substance called mucus on to their surfaces.
Mucous - A clear sticky semi-liquid secreted by cells in mucous membranes.
Pathogenic infection - An infectious organism which has the potential to cause disease. This is in contrast to the many organisms that live normally in or on the body which never cause disease and are called commensals.
Phagocytes - Cells of the body whose special task is to engulf bacteria, viruses, or parasites in an attempt to destroy them. They are also called macrophages.
Phagocytosis - The process whereby the specialised cells of the body engulf bacteria, viruses or parasites in an attempt to destroy them.
Plasma sample - A whole blood sample taken hygienically with a syringe and mixed with an anti-clotting agent so that it remains liquid. The sample is spun fast in a centrifuge and the red and white blood cells sediment to a firm pellet at the bottom leaving a clear liquid - the plasma.
Serology - Tests done in the laboratory to detect the level of specific antibodies in serum samples. ("ology" means study of - so literally serology means "study of serum").
Serum sample - A whole blood sample taken hygienically with a syringe and allowed to clot. The serum is the clear straw-coloured liquid which can be drawn of with a pipette. It contains the antibodies.
Titre - The concentration of a specific antibody in a serum sample. It is expressed as the amount by which the serum has to be diluted before a serological test goes negative.
Virulence - How pathogenic an organism is. Organisms with a high capability of causing disease are called highly virulent.