Recognize the cells and organs of the immune system and their functions. The key primary lymphoid organs of the immune system are the thymus and bone marrow, and secondary lymphatic tissues such as spleen, tonsils, lymph vessels, lymph nodes, adenoids, and skin and liver. Leukocytes (white blood cells) act like independent, single-celled organisms and are the second arm of the innate immune system.
Timing. Your dog can come into her first heat any time between 6 and 24 months. According to Webvet.com, small dogs come into heat sooner than larger breeds. Typically she will have two estrus cycles, or heats, a year, and each one lasts for between 18 to 24 days.
How Long Does A Dog's Period Last? A dog getting her period is usually referred to as the dog âgoing into heat.â It usually occurs about twice a year, with the first cycle coming sometime between the time the dog reaches six months of age and her first birthday. Each cycle will last anywhere from 1 to 4 weeks.