Active immunity: Immunity produced when an animal’s own immune system reacts to a stimulus e.g., a virus or bacteria, and produces antibodies and cells which will protect it from the disease caused by the bacteria or virus. Compare with 'passive immunity.'

Adjuvant: A substance added to killed vaccines to stimulate a better immune response by the body. Common adjuvants contain aluminum compounds.

Anamnestic response: The faster and greater immune response produced by an animal who has previously encountered that specific antigen. Memory cells are responsible for this more efficient response. Also called 'secondary response.'

Antibody: Small disease-fighting proteins produced by certain types of cells called 'B cells.' The proteins are made in response to 'foreign' particles such as bacteria or viruses. These antibodies bind with certain proteins (antigens) on foreign particles like bacteria, to help inactivate them. See also antigen.

Antibody Titer: A measurement of the amount of antibodies in the blood. The test to measure antibodies is usually performed by making a number of dilutions of the blood and then measuring at what dilution there is sufficient antibody to react in the test. For example, a titer of 1:8 (one to eight) means the blood can be diluted to one part blood and seven parts saline and still produce a positive reaction in the test. The higher the titer (1:16 is higher than 1:8), the more antibody is present.

Antigen: A molecular structure on surfaces of such particles as bacteria and viruses. This structure is recognized by the body as 'foreign' and stimulates the body to produce special proteins called antibodies to inactivate this foreign invader. It also stimulates special lymphocytes which directly kill the foreign invader, or release special chemicals which activate macrophages to kill the invader. See also antibody.

Attenuated: Weakened. An attenuated virus is one which has been changed such that it will no longer cause disease. An attenuated virus would be used in a modified live vaccine.

B cell: Also called 'B lymphocyte.' The type of lymphocyte which produces antibody. Compare with 'T cells.'

Cell-mediated immunity: The immunity that is the result of either special lymphocytes directly killing the foreign invader, or lymphocytes (T cells) releasing special chemicals which activate macrophages to kill the invader. Compare with 'humoral immunity.'

Core vaccine: Vaccine which should be given to all animals of certain species, example, parvovirus vaccine in dogs or panleukopenia in cats (see noncore vaccine).

Corticosteroid: Hormones produced by the adrenal gland which are important to almost every function of cells and organs. They are divided into two groups: glucocorticoids and mineralocorticoids. Glucocorticoids regulate protein, carbohydrate, and fat metabolism. Mineralocorticoids regulate electrolyte balances.

Duration of immunity: Length of time an animal is protected from a disease. Vaccines for some diseases provide long durations of immunity (years), while vaccines for some other diseases only provide immunity that lasts for 6 months.

Glucocorticoid: Hormones produced by the adrenal gland which regulate protein, carbohydrate, and fat metabolism, and are important to almost every function of cells and organs. They also stabilize cell membranes which is an important part of their function in treating allergic reactions. Also called glucocorticosteroids.

High titer vaccine: A modified live vaccine that contains a higher number of virus particles than the 'average' vaccine. High titer vaccines can generally elicit an immune system response in young animals who have a maternal antibody level that would prevent them from responding to an 'average' vaccine.

Humoral immunity: The immunity that is the result of antibody production by B cells. Compare with 'cell-mediated immunity.'

Immune-mediated reaction or disease: A condition or disease caused by abnormal activity of the immune system in which the body's immune system either overreacts (e.g., immune-mediated contact dermatitis) or starts attacking the body itself (e.g., autoimmune hemolytic anemia).

Immune system: The body's defense system which recognizes infectious agents and other 'foreign' compounds (such as pollen), and works to destroy them.

Immunity: A condition in which the animal's immune system has been primed and is able to protect the body from a disease-causing agent such as a certain virus or bacteria. An animal could have immunity to one agent, such as parvovirus, but not have immunity to another agent, such as rabies.

Immunization: The process of rendering an animal protected (immune) against a certain disease. Vaccination is a way to produce immunization. However, just because an animal has been vaccinated (received a vaccine) does not necessarily mean the animal is immune. If the body did not correctly react to the vaccine or if the vaccine was defective, immunity would not occur. No vaccine produces immunity in 100% of the population to which it was given. 'Vaccination' is not the same as 'immunization.'

Inflammation: A condition in which tissue reacts to injury and undergoes changes during the healing process. As an example, a toe with a sliver of wood in it would be inflamed and show the signs of inflammation which include redness, increased temperature, pain, swelling, and a loss of or disordered function. The toe is swollen, red, hot, painful, and the animal is reluctant to walk on that toe.

Intramuscular: Into the muscle (IM).

Intranasal: Into the nose.

Killed vaccine: Vaccines which are made by taking the real, disease-causing viruses (or bacteria), killing them, and putting them into a liquid base. Compare with 'modified live vaccine' and 'recombinant vaccine.'

Low passage vaccine: A low passage vaccine contains virus particles which have been attenuated, or weakened, less than those in the 'average' vaccine. Low passage vaccines can generally elicit an immune system response in young animals who have a maternal antibody level that would prevent them from responding to an 'average' vaccine.

Lymphocytes: The class of cells in the body which are responsible for mounting an immune response. Two main types are B cells and T cells.

Lymphokines: Chemicals produced by T lymphocytes. Some lymphokines signal macrophages and other phagocytes to destroy foreign invaders.

Macrophage: A type of phagocyte (cell in the body which 'eats' damaged cells and foreign substances such as virus and bacteria).

Maternal antibody: Antibody in a newborn animal which the newborn acquired through the placenta or colostrum (the first milk).

(Immunologic) Memory: When an animal mounts an immune response against a foreign substance, some cells are created to 'remember' the antigens on that substance. If the animal is again exposed to the substance, these cells will help the body respond much faster and to a higher degree.

Modified live vaccine: Vaccines which are made by taking the real, disease-causing virus and altering (attenuating) it in a laboratory to a non-disease causing virus. Compare with 'killed vaccine' and 'recombinant vaccine.'

Monovalent vaccine: A vaccine that is manufactured to stimulate the body to produce protection against only one disease, e.g., rabies vaccine. Compare with 'multivalent vaccine.'

Multivalent vaccine: A vaccine that combines two or more components to stimulate the body to produce protection against all the components. Most 'distemper' vaccines for puppies are of the multivalent type, and commonly include distemper, parvovirus, adenovirus cough, hepatitis, and parainfluenza. Compare with 'monovalent' vaccine.

Noncore vaccine: Vaccine which should only be given to animals at increased risk of exposure to a disease, example, leptospirosis in dogs or feline leukemia in cats (see core vaccine).

Nonpathogenic: Not causing disease. Some bacteria, such as those that normally live in an animal's intestines, are nonpathogenic.

Passive immunity: Immunity produced by providing an animal with antibodies or immunologic cells from another source, such as colostrum. Compare with 'active immunity.'

Pathogenic: Causing disease.

Phagocyte: Cell in the body which 'eats' damaged cells and foreign substances such as virus and bacteria. A macrophage is a type of phagocyte.

Recombinant vaccine: There are certain antigens on viruses and bacteria which are better at stimulating an antibody response by the animal than others. The genes for these antigens can be isolated, and made to produce large quantities of the antigens they code for. A recombinant vaccine contains these antigens, not the whole organism. Compare with 'modified live vaccine' and 'killed vaccine.'

Secondary response: The faster and greater immune response produced by an animal who has previously encountered that specific antigen. Memory cells are responsible for this more efficient response. Also called 'anamnestic response.'

Shedding (of organisms): A term used to describe the release of organisms (bacteria, protozoa, viruses) into the environment from an infected animal. The organisms may be in the stool, urine, respiratory secretions, or vaginal discharges. The 'shedding' animal may or may not be showing symptoms of disease.

Subcutaneous: Under the skin; often called 'sub Q.'

T cell: Also called 'T lymphocytes.' The type of lymphocyte which is responsible for cell-mediated immunity. T cells may directly kill a cell or produce chemicals called lymphokines that activate macrophages which will kill the cell. Compare with 'B cell.'

Titer: A measurement of the amount of antibodies in the blood. The test to measure antibodies is usually performed by making a number of dilutions of the blood and then measuring at what dilution there is sufficient antibody to react in the test. For example, a titer of 1:8 (one to eight) means the blood can be diluted to one part blood and seven parts saline and still produce a positive reaction in the test. The higher the titer (1:16 is higher than 1:8), the more antibody is present. (NOTE: The word 'titer' may also be used when discussing the amount of antigen present, e.g., a high titer vaccine has a large number of virus particles.)

Vaccination: The act of giving a vaccine. See also 'immunization' since the two words have different meanings and are often confused.

Vaccine failure: A term often used to describe a condition in which an animal who was vaccinated against a disease still gets the disease. In truth, there is usually nothing wrong with the vaccine, but for some reason, the animal's immune system did not adequately react to it.

Virus: The smallest form of life, invisible with an ordinary microscope. An infectious unit that enters and uses cells of plants or animals for replication. Some viruses cause disease in animals or plants.

Window of susceptibility: A time period in the life of a young animal in which the maternal antibodies are too low to provide protection against a certain disease, but too high to allow a vaccine to work and produce immunity.