The Immune System and Immunity

In this article you will find all you need to know about the immune system and titre tests – how they work, and how to interpret Titre Test results. Throughout the article you will see words which may seem rather scientific - find these in the definitions list below!

Useful definitions for this article;

- Antibody: a Y-shaped protein in the blood which attaches to, and destroys or neutralises pathogens by blocking them from attaching to cells in the body, leading to disease.

- Antigen: foreign bodies or toxins

- B-Lymphocytes (B Cells): a type of white blood cell responsible for creating antibodies.

- Effector Cells: differentiated from B-Lymphocytes, these cells secrete antibodies for the specific antigen/pathogen it’s targeting.

- Memory Cells (Memory B Cells): differentiated from B-Lymphocytes, these cells hold ‘data’ to remember the antibody shaped to deal with a specific Antigen/Pathogen

- Pathogen: an organism causing disease to its host, such as a virus or bacteria

- Seroconversion: specific antibody development following either infection or immunisation, whether that be through vaccines, or gentle exposure to the pathogen.

- T Helper Lymphocytes: cells which help the differentiation of B-Lymphocytes.

How the Immune System Works

Types of immunity;

* Active Immunity: this is what is happening when the dog is exposed to a pathogen. The active immune system is the fastest acting system and is the body’s first response to the presence of a pathogen. In the presence of a pathogen, the B-Lymphocytes create and release antibodies.

* Passive Immunity: this is the immunity passed on to an individual instead of being created by their body. In dogs we call this Maternally Derived Antibodies (MDA). MDA is passed onto the puppies from the mother and is vital to health in the early weeks. It usually wanes between 10 and 16 weeks of age, unless interrupted by vaccination. MDA is passed onto the puppies through the placenta, and through the mothers milk. If a puppy is hand reared, he/she may require extra caution than those who drink mothers milk.

* Innate (also called non-specific) Immunity: this is immunity and defence systems your dog is born with. Barriers like skin, the gastro-intestinal tract, eyelashes etc all help keep pathogens out of the body which is why gut health is so very important – at least 70% of your immune system is in the gut! Defence systems like mucosal layers, saliva, stomach acid etc are also part of the innate immune system. Another immune response included as part of the innate immune system is inflammation – this often acts as a marker to pathogens so they can be destroyed.

* Adaptive (or Acquired) Immunity: this is the immunity gained following seroconversion of vaccinations. The Adaptive Immune System can be separated into two mechanisms.Humoral (antibody mediated) immunity primarily involves B-Lymphocytes. During a humoral immune response, when an antigen is detected, with the help of T Helper Lymphocytes, the B Cells go through a differentiation process, which produces Memory B Cells and Effector B Cells, which are both specific to the B Cell they were differentiated from, and therefore are specifically shaped to combat a specific antigen/pathogen. This is the most common immune response, especially following successful vaccination. Titre Tests pick up these antibodies. Cellular, or cell-mediated immunity involves another type of cell – T-Cells. When T-Cells differentiate, they become T-Killer

Cells which attach to and engulf antigens. Allergic responses and autoimmune conditions are part of the cell-mediated adaptive immune system.

What Happens When the Body is Challenged?

* Challenge stimulates the body to release the antibodies required to match the shape of the viral particles of the present pathogen.

* When the B-Lymphocytes detect the presence of the pathogen, the T Helper Lymphocytes cause the B-Lymphocytes to differentiate into Memory B and Effector B cells, as described above under the humoral immune system.

* When the Memory B Cells are triggered, they ‘remember’ which shape the Effector cells need to be to attach to the pathogen through the Binding Site. The pathogen/antigen will be a specific shape – each pathogen for each different virus or bacteria will be a different shape. The different shaped parts are called ‘markers’. Antibodies are Y shaped. The ends of the top of the Y are where the Binding Site is located. The Binding Site will be a complementary shape to the markers, allowing it to attach perfectly.

* Once the binding site has attached to the antigen/pathogen, the cell can engulf it, and destroy it.

* The Effector cell also releases free antibodies which circulate in the immune system to pick up and target any free pathogens they may come across. Memory cells also have antibodies attached but these are membrane bound so they cannot release antibodies into the bloodstream

– the effector cells do the bulk of the work... memory cells simply remember which viruses they have seen before and rapidly trigger the secretion of antibodies from effector cells.

Types of Titre Test

As you will read in our article Vaccinations, Immunisation, Titre Testing and WSAVA Guidelines, Titre Testing is a way to check for circulating antibodies. There are a couple of types of titre test:

* The Vaccicheck test is the cheaper option, and uses an ‘Immunocomb’ to provide results – this is done ‘in-house’ at the vet. The comb has 12 ‘prongs’, so tests up to 12 blood/serum samples at a time and gives readings for circulating antibodies of the three core viruses – Parvovirus, Infectious Hepatitis (also called Adenovirus), and Distemper. The kit comes with a ‘developing pate’, in which the samples are put into the wells where the appropriate reagents for developing results are found. The comb is then inserted into the plate, and left for the time stated on the instructions before being moved back a row on the plate, and this is repeated from rows A to F.

Once this sequence is complete, the comb is removed, left to dry and then read. The darker the spots on the comb, the higher the number of circulating antibodies detected. The results are qualitative – up to the observation of the reader who may then send the results in a numerical form, which may be up for scrutiny depending on who reads the results and how they interpret the varying degrees of colour intensity.

* Numerical values for titre testing can be sought by having your vet send your dog’s blood sample to a laboratory. These tests involve serial dilution (each called a ‘titration’). At the end of the test, the lab technician will be able to write down a score for the antibody level for each disease – these are usually between 0 and 5, 5 being the highest amount of antibodies. Generally, on the reports, the results show as a number and ‘positive’ or ‘negative’ in brackets – 0-2 is usually recorded as ‘negative’, 3-5 as ‘positive’... HOWEVER we know these are purely a snapshot of the moment in time the blood sample was taken, and we know from earlier in this blog, antibodies are released by challenge! So how do we interpret titre tests properly? Read on to find out!

Reading Titre Results

Sadly, a lot of the result sheets state a score of 0-2 is ‘negative’ and recommend re-vaccination. Unfortunately, this could still cause over-vaccination. The low score, or score of zero could be due to two things: lack of challenge, and the dog’s lack of ability to seroconvert the vaccinations given; known as ‘non responders’. When the body hasn’t been challenged by the virus in question, it’s because the dog hasn’t been walked somewhere where the virus is prevalent prior to blood sampling or hasn’t been in contact with a dog with/shedding the virus (which is rare anyway). Canine Infectious Hepatitis and Distemper are really quite rare throughout the UK, whereas parvovirus is more common in the wild. This is often reflected in titre results, especially those we see in the group. Prevalence of these viruses will of course differ in other countries.

NO CHALLENGE = NO/LOW ANTIBODY LEVEL = 0-2 TITRE SCORE ... 0-2 TITRE SCORE DOES

NOT EQUAL NO IMMUNITY!

When dealing with a non-responder, we have to remember that if that individual fails to seroconvert/make use of the vaccine, no amount of further vaccinations will help! In these situations, gut health is the most important thing we can do to help these dogs – keep the gut healthy, keep disease at bay.

FAILURE TO UTILISE THE VACCINE = NO DETECTABLE ANTIBODIES ... BUT DOES NOT MEAN

FURTHER VACCINATION IS REQUIRED

You can read more on the longevity of vaccinations and view research from canine immunologist Dr Ronald Schultz in the Vaccinations, Immunisation, Titre Testing and WSAVA Guidelines blog and the Minimal Puppy Vaccination Protocol article. Titre tests are useful for situations whereby proof of immunity is required for activities or acceptance into dog kennels or boarding. Walking in high dog populated areas is a good idea prior to blood sampling to ensure a higher chance of there being circulating antibodies to be detected in the titre test. Some people prefer to do one test when their ‘first booster may be due’ according to their veterinary practice then no further tests, others prefer to do them every few years just out of pure curiosity – this is a personal choice. Titres are a great way to prove a vaccine has worked and is far better than needlessly giving vaccines which can lead to numerous health issues.

Useful link: https://vitalanimal.com/fallacy-of-titer-tests

Leptospirosis, and why it’s not included

Leptospirosis is different to Parvovirus, Distemper and Hepatitis because where the core three are viral, Leptospirosis is bacterial. The immunity provided for bacterial infections by vaccines is much different to that provided by vaccines for viruses. The duration of immunity is proven to be extremely low in comparison, which is why vets recommend giving that one annually (though we don’t share the same opinion – i’ll add some interesting reading links below regarding the leptospirosis vaccines and their efficacy, ability to shed to the environment, and rates of reaction).

The only time antibodies will be present enough to be detected by a blood titre test for bacterial diseases would be when the body is actively fighting off a bacterial load.

https://www.mypetnutritionist.com/post/facts-on-the-leptosporosis-vaccine

Written by Bethany Calhoum