Lymph nodes are present throughout the body. They are clusters of different types of immune system cells that help in combating infections in the body. Several lymph nodes of your pet are assessed for their normal size and texture by your veterinarian. Any tumor, Viral, bacterial and fungal infection can cause swelling and inflammation of these lymph nodes. In case of detection of a swollen lymph node, a sample can be drawn for further evaluation under the microscope. If this does not result in a conclusive diagnosis, usually a biopsy is performed.