Why does the reaction to the vaccination happens and why does a lump form?

The reaction can be directed against the infectious organism, vaccine stabilizers, preservatives or residue from the vaccine organism’s laboratory tissue culture. Each immune system reacts differently to the offending protein.

The makeup of a vaccine includes an infectious organism rendered harmless but still capable of stimulating the immune system: in some instances, the organism is live, but weakened in a way, in other instances, the organism is killed. 

The larger the organism and the more complicated the proteins, the more local inflammation can result after the injection, and this sometimes translates into a firm knot or bump at the injection site.  These growths are typically noted a few weeks following vaccination and generally resolve on their own. These are not allergic reactions but are inflammatory reactions caused by the immune stimulation. 

What to do if your pet develops a lump?

The lump will usually resolve on its own. If a lump persists longer then three months from the time of vaccination or is larger than 2 cm in diameter any time after vaccination, then it should be removed and biopsied. This removal is particularly important for cats as they have the potential to develop injection site tumors.

Sad fact: Do you know that as per the studies approximately one in 250 dogs will have some sort of vaccination reaction?