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 gestation or pregnancy period in dogs begins from the first breeding session or fertilization until the birth of the puppies. Though the period may vary as per the size or breed of the dog, usually, the gestation for small dogs as well as big dogs lasts for about 60 to 63 days, or 9 weeks. Female dogs have menstrual cycles throughout their lives and are capable of reproducing for as long as they live.