In Dairy cattle changes in metabolism and lack of optimum immunity have been closely associated with most of the transition period diseases. During transition period the function of immune system is compromised and some pathogens like Staphylococcus aureus or Streptococcus uberis can evade local defences and establish chronic infection while sometimes the function of immune system is uncontrolled for e.g. in coliform mastitis in early lactation. At the time of pathogen exposure immune cells and chemical mediators act in a coordinated way to result in optimal disease resistance that means at the time of exposure to pathogens inflammation starts and immune and non-immune cells recognize the pathogen then activate cell populations and release pre inflammatory signalling molecules. In transition period, every high yielding cow suffers from varying degrees of subclinical hypocalcaemia due to utilization of calcium in formation of foetal bones as well as drain of calcium in milk and colostrum. So we can say that the optimum balance between sufficient immune response and return to normal immune homeostasis is in transition period.