How a B cell is activated

A B cell switches on because of what it carries on its surface. That is where its receptor sits, an antibody anchored in the membrane, and it is the receptor that recognizes the antigen, the piece of microbe it is matched to. Other pieces on that same surface are in charge of carrying the message inward.

And they need to be, because that receptor has a very short tail inside the cell and cannot pass the word along by itself. It is an antenna that picks up the signal but needs a cable to carry it inside: that cable is two proteins sitting alongside it, Ig-alpha and Ig-beta.

Next to them are coreceptors, such as CD21 and CD19, that work like amplifiers: they lower the threshold, so less antigen is enough for the cell to be stimulated through that receptor.

Once switched on, the cell multiplies in the germinal center, the area of a follicle where B cells multiply and specialize. There, the cytokines released by a helper T cell shape what it ends up becoming: a plasma cell or a memory cell.

Of the two, the one that does the manufacturing is the plasma cell: it is the one that secretes antibodies tailored to that antigen.