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.
The proestrus is relatively long at 5â7 days (9 days), while the estrus may last 4â13 days, with a diestrus of 60 days followed by about 90â150 days of anestrus. Female dogs bleed during estrus, which usually lasts from 7â13 days, depending on the size and maturity of the dog.