Pesticides are chemicals designed to combat the attacks of various pests on agricultural and horticultural crops. They fall into three major classes: Insecticides, fungicides, and herbicides. There are also rodenticides for vertabrate pests, nematicides for microscopic eelworms, molluscides for slugs and snails, and acaricides for mites. These groups have been subdivided according to their chemical structures. There are a number of possible ways in which humans can be exposed to pesticides, thus the toxic effects of pesticides may have consequences for general public consuming foods as well as farmers and other pesticide applicators. Pesticides used domestically in wood preservation or as household insecticides may be also important sources of exposure furthermore, pesticides may contaminate water supply. Because of the widespread use of pesticides for domestic and industrial applications, the evaluation of their immunotoxic effects is of major concern. A number of experimental data have provided evidence that acute and chronic exposure to pesticides may result in alterations in immune parameters in animals and in vitro test systems. Functional changes included altered antibody production, impaired antigen presentation by macrophages, increased or decreased responsiveness to recall antigens, changes in the proliferation, cytolytic activity, and number of natural killer cells. However the accumulated data are not consistent and the differences can be accounted for by variations in species, strains, dosages, routes and the duration of exposure to different pesticide compounds. And in the majority of experiments, exposure of the animals was to a high dose of pesticide over a short period, and in some cases, the doses used were frankly toxic. In humans, very few and limited data about the influence of pesticides on the immune system are available, but it was also suggested that immunotoxic abnormalities induced by occupational and environmental pesticide exposure may exist as well. There are only a few reports of hypersensitivity caused by exposure to pesticides and also there is little evidence that pesticide exposure has led to an increased risk of developing infection or cancer. In this review, the effects of insecticides on the immune system parameters have been discussed.