Fig. 5.10 Subsets of helper T (TH) cells. In response to stimuli (mainly cytokines) present at the time of antigen recognition, naive CD4+ T cells may differentiate into populations of effector cells that produce distinct sets of cytokines that act on different cells (indicated as target cells) and mediate different functions. The roles of these subsets in host defense and immunologic diseases are summarized. These populations may be capable of converting from one to another. Some activated T cells produce multiple cytokines and do not fall into a distinct subset.
Functions of antibodies
Neutralization of microbe and toxins
Naive IgM+, and phago-IgG cytosis
High-affinityT cells, IgGother stimuli
Complement Lysis of activation microbes
Fig. 5.11 Humoral immunity. Naive B lymphocytes recognize antigens, and under the influence of Th cells and other stimuli (not shown), the B cells are activated to proliferate and to differentiate into antibody-secreting plasma cells. Some of the activated B cells undergo heavy-chain class switching and affinity maturation, and some become long-lived memory cells. Antibodies of different heavy-chain classes (isotypes) perform different effector functions, shown on the right. Note that the antibodies shown are IgG; these and IgM activate complement; and the specialized functions of IgA (mucosal immunity) and IgE (mast cell and eosinophil activation) are not shown.
T cell–independent. Many polysaccharide and lipid antigens have multiple identical antigenic determinants (epitopes) that are able to simultaneously engage and cross-link several antibody molecules on each B cell and initiate the process of B-cell activation.
T cell–dependent. Typical globular protein antigens are not able to bind to multiple antigen receptors, and the full response of B cells to protein antigens requires help from CD4+ T cells. B cells also act as APCs—they ingest protein antigens, degrade them, and display peptides bound to class II MHC molecules for recognition by helper T cells. The helper T cells express CD40L and secrete cytokines, which work together to activate the B cells.