Cells of the immune system are present as circulating cells in the blood and lymph. They are distributed to almost every organ and tissue of the animal body. Their distributions upon exposure to an external agent or pathogens are utmost important in the generation of effective immune response. At beginning, the immune system must respond to the entering pathogen followed by an adaptive immune response by specific lymphocyte. Finally, the cells of adaptive immune response destroy the pathogens by the effector cells.