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.
From about six months old to through the rest of her life, a female dog will experience estrus, or heat, roughly every six months.eat usually lasts between 2-4 weeks. Early in the cycle, a female dog may not be receptive to male dogs, although some are receptive through the entire cycle. It can be shorter or longer and youâll know the cycle is over when all her vulva returns to its normal size and thereâs no more bleeding or discharge.