What does it mean when a part of the immune system does not function properly?

Our immune system is made up of special cells, tissues, and organs that work together to protect against foreign microbes. When a part of the immune system does not function properly, it might lead to a condition called low immune system. It is nothing but weakness in the body’s natural ability to fight against diseases. People with low immunity show frequent colds, more susceptibility to flu and more serious illnesses.

i'd say that the moon rock is about 2.2 billion years old.

the answer is d.) homologous; have mammary glands