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.
Canine Estrus Cycle. If your female dog isn't spayed, expect her to enter estrus approximately every six months. If she's a small breed, you might have to deal with the bloody, smelly discharge accompanying her heat cycle three times annually. Usually, a canine heat cycle lasts for two to three weeks, although the dog won't bleed that entire time. The initial discharge is fairly heavy, but it tapers off during a seven- to 10-day period. In addition to bleeding, a dog in season also urinates more frequently.