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.
1 On average, dogs have two cycles a year. 2  The estrus cycle lasts on average 12-21 days, but maybe be as short as a few days to four weeks. 3  The estrus period length varies widely between breeds and individual dogs. 4  The length of a cycle varies widely, even for dogs of the same breed.ome general rules of thumb for canine estrus: 1  The first estrus cycle usually occurs by age 6-12 months; for some small breeds, as early as 5 months, and for some large and giant breeds, the first cycle may not occur until 14 months of age or older. 2  On average, dogs have two cycles a year.