Basophils
Basophils, seen here, are a very rare granulocytes population comprising less than 1% of circulating leukocytes. They are named for the susceptibility of their large granules to be stained by basic dyes. Interestingly their function is still relatively unknown. In many ways they share characteristics to mast cells, like their expression of FceRI and their ability to release large amounts of histamine after FceRI cross-linking. So are basophils really required if mast cells are already doing the same job? Well yes! There are also many differences between basophils and mast cells. Their anatomical location differs as mast cells are tissue resident cells and basophils circulate in the blood until attracted into inflamed or infected tissues. Which cell do you think would release histamine first after detecting a pathogen in the blood; the mast cell or the basophil? Most likely the basophil! Basophils not only get activated via FceRI but also have TLRs to detect various different pathogens and Fc-receptors specific for other kinds of antibodies like IgG. Thinking of their location in the blood it is not surprising that basophils are central in IgG-mediated anaphylaxis which is a widespread, many times systemic, severe allergic reaction! Another very different characteristic of basophils is that they pre-store huge amounts of the Th2 cytokines IL-4 and IL-13. Mast cell has to make these once activated. Later you will learn how these two cytokines, especially IL-4, drive differentiation of naive T cells to become Th2 cells and influences B cells to produce IgE. Both these processes are critically important for starting an allergic immune response. It may be that basophils provide the right environment for this by their early burst of IL-4.
But in most allergic diseases basophils are not perceived as quick players at all! Why not? They have pre-stored mediators that are quickly released, so what's the problem? Well, IL-4 and IL-13 also upregulate the adhesion molecule VCAM-1 on the endothelium which aids "leukocyte recruitment" into tissues? That's it! The basophil needs to be attracted into the allergic tissue by cytokines and chemokines before it can play its part in the late phase allergic response. So maybe we should summarise the basophil as quick but slow!