The immune system is a complex network of specialized cells and organs designed to defend a dog's body against bacteria, viruses , toxins , parasites and any foreign material that invade the dog’s body. Millions of different types of immune cells pass information back and forth, which results in a protective system that is always ready to produce an immune response that is fast and effective. The immune system is also a component of the lymphatic system. Dogs use their senses to adapt to new environments as well as putting to use thousands of years of selective breeding and training. Dogs are sexual not asexual, a female dog usually has 10 teats, though this does not mean she can not necessarily provide sufficient nutrition for 10 puppies in one litter. An average litter consists of about five to six puppies, even though this number may vary widely based on the breed of dog. Size of the breed is correlated with litter size. Toy to Small Breeds – weight at 6 weeks x 4 = adult weight.Medium to Large Breeds – weight at 14 weeks x 2.5 = adult