As I sat down to write this week, I looked over last week’s post. I had a good start on immune system function, but I’d like to dig a little deeper this week. I’ll try to keep things reasonably understandable. I’m also going to repeat some material so that I can expand the discussion. I will focus on two important things this week: how much vaccine we give pets, and the maternal antibody problem.
Activating the Immune System
When we physically inject a vaccine into a pet, we’ve placed a certain amount of viruses/bacteria/proteins into the body. This amount has to be enough to stimulate the immune system to fully respond to the vaccine. We need the various types of white blood cells to show up and process the pathogens/infected cells so that we get a robust humoral and cell-mediated response.
I frequently get asked about whether I give the ‘same amount’ of a vaccine to a small dog as I do to a big dog. Without a doubt, yes, we do give the same amount of vaccine to a Chihuahua as we do to a Great Dane. The reason is that the immune systems of both of those dogs work the same way. There is a minimum amount of pathogen required to generate a robust immune response. If we give less, the vaccine may not provoke the immune system enough, and we could leave that pet unprotected. Immune responses vary a little from individual to individual. Manufacturers have figured out how much pathogen we need for the vast majority of pets to respond, and that’s what’s put in the vaccines. If we gave less to a smaller dog, the response might not be enough to protect from disease. The one exception to this “rule” is the nasal bordetella vaccine. We’re able to lose/spill some of that and still get a robust response in the pet.
I mentioned last week that when we adequately stimulate the immune system, two pathways are activated. The two parts of the system are Humoral and Cell-Mediated. Humoral immunity is composed of antibodies circulating in the bloodstream or standing guard on surfaces such as the lining of the nasal passages. Some antibodies are attached to other immune cells and function as on/off switches for those cells’ functions. Antibodies are also known as immunoglobulins, abbreviated as “Ig.” Immunoglobulins have a neat structure that looks like a “Y.”
The structure of the antibody molecule has a “socket” shape on the upper tips of the ‘Y’ shape. The socket fits a particular bump on a pathogen. So, there are antibodies with a socket that fits a protein that the Lepto bacteria wears on its surface. For each pathogen, the body makes several types of immunoglobulins (Ig): IgA, IgD, IgE, IgG, and IgM. For this part of the discussion, we’re going to focus on IgA, IgM, and IgG.
IgA is found on the surfaces in the nose, throat, intestinal tract, and urogenital tract. IgA functions on these surfaces to block pathogens from getting into these surface cells or tissues to cause an infection. Our Bordetella (Kennel Cough) nasal vaccine causes a high level of IgA to be produced on the respiratory tract surfaces.
IgM is an antibody secreted by certain types of white blood cell (B Cells), and it’s the first antibody produced when an animal is exposed to a pathogen. It’s a very large molecule, so it stays primarily in the bloodstream. IgM is very good at activating the Complement System, which is the chemical reaction that occurs when an Ig sticks to a pathogen. The Complement System functions to kill a pathogen by punching a hole in the pathogen.
IgG is found circulating in the bloodstream and in body tissues. IgG sticks to pathogens to stop them from getting deeper into the body and also aids other parts of the immune system in responding. IgG molecules can also cross the placenta and provide some immunity to a fetus. IgG is produced during later phases of the immune system’s response to a pathogen, or when the body is later exposed to a pathogen it has been previously exposed to. This is the antibody produced when we give a booster vaccination, and it’s what we count on to protect our pets.