The various cells and proteins responsible for immunity constitute the immune system, and their orchestrated response to defend foreign/non-self substances (antigen) is known as the immune response. When an antigen attacks the host system, two distinct, yet interrelated, branches of the immune system are active-the nonspecific/innate and specific/adaptive immune response. Both of these systems have certain physiological mechanisms, which enable the host to recognize foreign materials to itself and to neutralize, eliminate, or metabolize them. Innate immunity represents the earliest development of protection against antigens. Adaptive immunity has again two branches-humoral and cell mediated. It should be noted that both innate and adaptive immunities do not work independently. Moreover, most of the immune responses involve the activity and interplay of both the humoral and the cell-mediated immune branches of the immune system. We have described these branches in detail along with the mechanism of antigen recognition. This chapter also describes the disorders of immune system in brief.