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.
This is the reproductive phase in dogs and if the female dog undergoes intercourse at this time, she could get pregnant. The female dog menstruation period usually starts when she is 5 to 10 months old; however, in some small breeds, it may start at six months. The female dog menstrual cycle lasts for 21 days or three weeks on an average, but it may vary from 8 to 30 days. Most female dog heat cycle is experienced at least twice a year or once every seven months. The heat cycle or the canine estrous cycle is divided into the four stages mentioned below: