Are you sure though that pleasure doesn't affect the immune system? It seems logical that it would from an evolutionary point of view. Pleasure and its anticipation are forms of arousal like stress; downgrading non-essential or non-urgent systems is worth it for a chance at mating or a successful kill during a hunt. It seems to me that levels of inflammation and tooth decay increase in my body after several days of over-indulgence -- including activities like eating cake!