Is it really possible for someone to engage in a completely selfless act? A rational exploration of the question must lead to a negative rather then affirmative answer. For isn't it the case that no matter what action one engages in, it involves their selves and some kind of motivation on their part? So long as the individual in question could be said to have a motivation for acting, even if this motivation is a benevolent one, it cannot be said that they are acting in a truly selfless manner. So long as one has conciousness, so long as one is self-aware, one cannot truly be selfless. In order to be selfless one would have to cease to exist altogether, or by the very least enter an inhuman state in which one has no volition. But such a state of being is not how human beings work. It could only be used to describe non-concious forms of life such as a plant.
Altruism is not an objective description of human actions so much as a rationalization used to compel people into engaging in certain actions. It is certainly true that one could engage in an action that benefits another rather then oneself, but such an action could not take place without the deliberate concious effort and motivation of the individual in question. If a benevolent act towards another is truly voluntary, then it cannot be said to be genuinely altruistic because in such a scenario the individual actually percieves the act to be in their self-interest at least on a psychological level. That is, they desire to give to others. A genuine desire originates within the individual themself. Satisfaction is obtained upon the fulfillment of the desire, even if the desire is to fulfill a percieved positive obligation towards another.
So long as human beings act to remove a source of dissatisfaction, it could not be said that they act in a void of self-interest. They act in the persuit of satisfaction, which is their percieved self-interest. They employ means for the purpose of obtaining desired ends. The statement that humans act in self-interest on a fundamental level and that human beings are rational animals does not mean that humans will always make correct choices, that the ends they desire are necessarily logical and ethical, or that the means they employ in the persuit of such ends are the proper or most efficient ones toward obtaining their goals. It is merely a description of how human action works, that human beings are volitional creatures with goals and the capacity to choose among means for the purpose of obtaining their goals.