Analogies are hard. They can be useful in getting across some difficult science concepts, but one has to be aware of their limitations. I don't know much about immunology, but I'm sure there's a point where any parallels with football start to break down. I always had a hard time with football, myself; I always manage to get the names of the positions terribly confused.

In physics, too, a good teacher should be able to talk about rotational motion without relying solely on one's presumed automotive experience.

We were actually discussing analogies in my Scientific Communications Seminar during my REU. We were critiquing each other's papers, and were discussing things like how, quantum mechanically, electrons have a property called "spin" even though they're definitely not little balls spinning around. Good analogies can be really helpful, but if not carefully used and qualified, they can lead to serious misconceptions.