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.
How Long Does a Dog Stay In Heat? Dogs are usually in heat for 21 days. Female dogs have a cyclic sexuality that is independent of season unlike most animals. As for the male dogs, they do not have any cycles and are always ready for intimacy like most males of any species including human beings. Female dogs have a cycle of six months on average meaning they can be in heat twice a year. The heat cycle has three distinct stages. They include: Proestrus