Final answer:
Although the provided information does not specify a percentage, we can infer that the proportion of self-antigens is more than 75%, given our understanding of the immune system's requirements for self-tolerance.
Step-by-step explanation:
The estimate for the proportion of antigens that are self-antigen can be understood by considering the workings of the immune system. The human body has mechanisms to recognize and tolerate its own antigens to prevent autoimmune reactions. However, the materials provided do not include a specific percentage for self-antigens. Based on common biological understanding, one can surmise that the vast majority of antigens that the immune system encounters on the body's own cells are self-antigens, as these are integral to the normal functioning of the immune system. From this, one can conclude that the closest answer to the question presented in this information would most likely be that the proportion of self-antigens is more than 75% (option d), although it's important to note that the actual percentage may vary and the materials provided do not give a precise value to cite.