Page 336 - The Veterinary Laboratory and Field Manual 3rd Edition

Serology and immunology 305

Specific immunity is more ‘specialized’ than and either measure immunoglobulins (Ig) (pre-
non-specific immunity in that it is a response to dominantly IgG and IgM – see Figure 6.4a) or are
a specific ‘invader’, that is, a specific bacteria, designed to detect antigens.
virus, parasite or other foreign protein. Cells
involved in the specific response recognize and
remember individual invaders following anti- 6.2 Antigens and antibodies
genic presentation (Figure 6.2b) and a cascade
of events occurs to deal with these invaders antigen
and any other subsequent invader with simi-
lar antigenic components. In this context, an An antigen is an immunogenic substance, usually
‘antigen’ is a substance that when introduced of large molecular weight, which has a chemical
into the body stimulates the production of cell- grouping on its surface rendering it capable of
and or antibody-mediated immune responses. stimulating an immune response in the animal
Some bacteria share a similar antigenic ‘coat’ host. Proteins tend to be more immunogenic
so cross reactions may occur which can result than most carbohydrates although lipopolysac-
in false positive reactions to a given antigen, charides may stimulate a strong immunogenic
for example, Yersinia enterocolitica and Brucella response and are frequently used in the prepara-
abortus in laboratory serology tests. Cross reac- tion of ‘killed’ vaccines.
tions such as this may also confer a degree of If purified antigen is injected into an animal’s
immune protection if an animal is exposed body it causes an immune response to take place
to another pathogen with a similar antigenic resulting in the production of specific antibodies
profile. that are detectable in the serum a few days later,
The specific defensive response may broadly the time frame can be variable depending on the
be split into two types, although they are inter- antigen type, route of administration and dose as
linked with each other and with the non-specific well as the host’s previous immune status (that
response (Figure 6.2b and Figure 6.3). One is, a more rapid and higher antibody titre would
is called the ‘humoral’ or antibody-mediated be seen if prior exposure to the same antigen
and the other the ‘cell-mediated’ response. had occurred). This process is commonly used
The humoral response involves the produc- for the production of specific antisera (usually
tion of B-lymphocytes from bone marrow, using laboratory rabbits) for the purposes of
lymph nodes, liver and spleen. B-lymphocytes sero-typing and the production of test reagents
produce antibody specific to an antigen. The cell- and controls. However, not all antigens stimu-
mediated response involves the production of late a strong serological response, for example,
T-lymphocytes from the thymus. T-lymphocytes intracellular bacteria such as Mycobacterium bovis.
produce and respond to cell-mediators to facili- For these organisms, the cell-mediated immune
tate phagocytosis and cell destruction. Tests to response is evaluated (that is, the tuberculin
measure the cell-mediated response include the skin fold test).
tuberculin skin fold test which is used to detect
cattle exposed to Mycobacterium bovis. However,
although new assays are being developed to mea- antibody/immunoglobulins
sure the cell-mediated immune response (for
example, lymphocyte proliferation assays, IFNγ There are five classes of antibodies, or (lg) in
assay) most of the commonly used diagnostic mammals (Figure 6.4a). The serum antibody
assays are still based on serological screening found in birds is referred to as IgY rather than

Vet Lab.indb 305 26/03/2019 10:26