Our bodies are constantly challenged with various pathogens from the outside. We humans carry a defense mechanism to distinguish between self and non-self matters when invaded with such pathogens. Our body’s immune system is built on a complex interaction of lymphocytes and a highly sophisticated system is required to regulate the cognitive process. This article demonstrates the morphology and functions of lymphocytes, the process of mutual recognition, immunological memory, the expression of surface antigen in lymphocytes, and the ability to distinguish self and non-self matters. The defense system of living organisms have been created based on an intelligent blueprint. The complexity of the immune system is another example that substantiates the limitations of the evolution theory.