This is an excellent, concise treatment of this advanced subject. You're not going to have a question on the MCAT where you have to choose between selectins and integrins as an answer choice, but it would not be unusual to find yourself in a passage where you have to figure out what's going on.

This is advanced material, but it's an important subject. You don't need to memorize everything for the MCAT but be ready to find your footing if a passage drops you into this world.