Problem
A father is distressed because the doctor has told him he should take antibiotics and/or move out of the house while his child is sick with strep throat. He understands this is because he once had rheumatic fever, but he doesn't see why this means he needs to fear his own child. "I already had rheumatic fever," he says. "So I can't get it again." The thing is, rheumatic fever is an autoimmune condition in which antibodies attack cardiac muscle proteins that are coincidentally very similar in shape to the exogenous antigens found on a strep bacterium. Most people who get strep never have problems, but anyone who has had rheumatic fever once is extremely likely to have it again on subsequent exposures to Streptococcus pyogenes (which causes strep throat).
i. What is the significance of the protein shape?
ii. Why do the antibodies only pose a problem when a person is infected with S. pyogenes and not other times?