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.
It can take up to 2 years for a female dog to develop regular cycles. There is no season of the year which corresponds to a breeding season for domesticated dogs. How long does estrus last? Although this can vary with the individual dog, on average a dog will be 'in heat' for 2-3 weeks. The onset of estrus is characterized by swelling of the external vulva. Next, the dog will develop a vaginal discharge.