They may experience irritability and depression because when a person constantly smokes, his/her brain begins to rely on the chemicals it gets from the person smoking, which results in the brain "getting lazy" and not producing the chemicals anymore. If the person were to suddenly quit, the brain would lack in the chemicals it had stopped producing, making the person feel constantly annoyed and depressed because of the chemical imbalance.
Having positive feedback about your good deeds or accomplishments, even the little things, is a good boost of confidence.
T cells attack antigens on foreign substances and B cells produce antibodies.
