B- and T-Cell Activation – Lymphocyte Activation

You’ll now look at the way in which a signal is sent into the B-cell in order to achieve B-cell activation. Along with the transmembrane version of antibody on the surface of the B-cell, there are the Ig-alpha (Ig-α) and Ig-beta (Ig-β) molecules with their cytoplasmic tails containing Immunoreceptor Tyrosine-based Activation Motifs or ITAMS. But that isn’t the entirety of the B-cell receptor because it’s also associated with other molecules. It’s associated with a B-cell co-stimulatory complex, and this consists of several different molecules. Leu13, CD21 which is also actually referred to as complement receptor 2 because it is indeed a complement receptor, CD19 which again importantly has ITAMs in its cytoplasmic tail, and CD81. So these four molecules, Leu13, CD21, CD19 and CD81 form the B-cell co-stimulatory complex, which together with the transmembrane version of antibody and the Ig-α and the Ig-β molecules are required for B-cell activation. If the B-cell has an antibody on its surface that is capable of recognizing a particular antigen; so let’s say that this is a Cytomegalovirus and the B-cell encounters Cytomegalovirus, then what happens is that the antibody will bind to the antigen that it is specific for and more than one B-cell receptor molecule will link together, be cross linked by the antigen. They’ll come together, and this will drag those ITAMs in Ig-α and Ig-β nearer to each other. And that cross linking of these antibodies by antigen, and therefore the aggregation of the Ig-α and Ig-β together with the B-cell stimulatory complex will lead to protein kinases such as Lyn kinase phosphorylating the tyrosines on the Immunoreceptor Tyrosine-based Activation Motifs. And this phosphorylation by kinases will initiate a signaling cascade that ultimately results in activation of the B-lymphocyte. So that’s how B-lymphocytes become activated. What about T-lymphocytes? Well, it’s...