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.
Small breed dogs typically start having estrus (heat) cycles earlier, at 4-6 months of age, while large breeds typically start later, at 9 to sometimes as late as 24 months of age. Estrus cycles can vary between each individual dog, an average of 6 to 12 days. Some cycles can last as few as 2 days, while others can last as long as 21 days.1 Female dogs cycle throughout their life and do not experience menopause (reproductive cycles ceasing with age), like human females do.