antibody

An antibody is a protein made by a B cell that fits one specific antigen — a molecule on a foreign cell or particle that the immune system recognizes. The fit is the whole idea: an antibody's specificity comes from its shape, and an antibody that fits one antigen does not fit another. Its main job is to mark and disable: it sticks to the antigen, flags it so a phagocyte can engulf it, and stops the antigen from working. That is why the first meeting with an antigen takes days — the body has to find and multiply the one B cell that fits — and why the second meeting is fast, because the memory cells left behind are ready to make the right antibody as soon as the antigen returns. Antibodies belong to the adaptive immune response, the specific and remembering one; the innate response works the same way on everything and keeps no memory.