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.
Best Answer: Technically, dogs don't have periods, It's called going into heat or going into season. The heat cycle of the female dog has three parts and lasts about 3 weeks. The first part lasts anywhere from 3-14 days during which a bloody discharge and swelling of the vaginal area are usually seen. Next the dog will become receptive to being bred and actively may seek out a male dog.