IgM is so called because of its size: although monomeric IgM is only 190 kDa, it normally forms pentamers, known as macroglobulin (hence the M), of very large molecular weight (see Fig. 4.23). IgA dimerizes to give a molecular weight of around 390 kDa in secretions. IgE antibody is associated with immediate-type hypersensitivity. When fixed to tissue mast cells, IgE has a much longer half-life than its half-life in plasma shown here. The activation of the alternative pathway of complement by IgA1 is caused not by its Fc portion but by its Fab portion. A subset of human V_H regions (clan 3, see Section 4-3) also binds staphylococcal Protein A through the Fab portion.