Helper T cells stimulate the progeny of IgM– and IgD expressing B lymphocytes to change the heavy-chain isotypes (classes) of the antibodies they produce, without changing their antigen specificities (Fig. 1). Different antibody isotypes perform different functions, and therefore the process of isotype switching broadens the functional capabilities of humoral immune responses. For example, an important defense mechanism against the extracellular stages of most bacteria and viruses is to coat (opsonize) these microbes with antibodies and cause them to be phagocytosed by neutrophils and macrophages. This reaction is best mediated by antibody classes, such as IgG1 and IgG3 (in humans), that bind to high-affinity phagocyte Fc receptors specific for the Fc portion of the γ heavy chain . Helminths, in contrast, are too large to be phagocytosed, and they are best eliminated by eosinophils. Therefore, defense against these parasites involves coating them with antibodies to which eosinophils bind. The antibody class that is able to do this is IgE, because eosinophils have high-affinity receptors for the Fc portion of the ε heavy chain. Thus, effective host defense requires that the immune system make different antibody isotypes in response to different types of microbes, even though all naive B lymphocytes specific for all these microbes express antigen receptors of the IgM and IgD isotypes.

Fig1. Immunoglobulin (Ig) heavy-chain isotype (class) switching. Antigen-stimulated B lymphocytes may differentiate into IgM antibody-secreting cells, or, under the influence of CD40 ligand (CD40L) and cytokines, some of the B cells may differentiate into cells that produce different Ig heavy-chain isotypes. The principal effector functions of some of these isotypes are listed; all isotypes may function to neutralize microbes and toxins. B cell–activating factor belonging to the TNF family (BAFF) is a cytokine that may be involved in switching to IgA, especially in T-independent responses. Switching to IgG subclasses is stimulated by the cytokine interferon (IFN)-γ in mice, but in humans it is thought to be stimulated by other cytokines. IL-4, inter leukin-4; TGF-β, transforming growth factor β.

Another functional consequence of isotype switching is that the IgG antibodies produced are able to bind to a specialized Fc receptor called the neonatal Fc receptor (FcRn). FcRn expressed in the placenta mediates the transfer of maternal IgG to the fetus, providing protection to the newborn, and FcRn expressed on endothelial cells and phagocytes plays a special role in protecting IgG from intracellular catabolism, thereby prolonging its half-life in the blood .

Heavy-chain isotype switching is induced by a combination of CD40L-mediated signals and cytokines. These signals act on antigen-stimulated B cells and induce switching in some of the progeny of these cells. In the absence of CD40 or CD40L, B cells secrete only IgM and fail to switch to other isotypes, indicating the essential role of this ligand-receptor pair in isotype switching. A disease called the X-linked hyper-IgM syndrome is caused by mutations in the CD40L gene, which is located on the X chromosome, leading to pro duction of nonfunctional forms of CD40L in males who inherit the mutation. In this disease, much of the serum antibody is IgM, because of defective heavy-chain isotype switching. Patients with this disease also have defective cell-mediated immunity against intracellular microbes, because CD40L is important for T cell– mediated activation of macrophages and for the amplification of T cell responses by dendritic cells .

The molecular mechanism of isotype switching, called switch recombination, takes the previously formed VDJ exon encoding the V domain of an Ig μ heavy chain and moves it adjacent to a downstream C region (Fig. 2). IgM-producing B cells, which have not undergone switching, contain in their Ig heavy chain locus a rearranged VDJ exon adjacent to the first constant region cluster, which is Cμ. The heavy-chain mRNA is produced by splicing a VDJ exon to Cμ exons in the initially transcribed RNA, and this mRNA is translated to produce a μ heavy chain, which combines with a light chain to give rise to an IgM antibody. Thus, the first antibody produced by B cells is IgM. In the intron 5′ of each constant region is a guanine-cytosine (GC) rich sequence called the switch region. Signals from CD40 and cytokine receptors stimulate transcription through one of the constant regions that is downstream of Cμ. During switch recombination, the switch region upstream of Cμ recombines with the switch region adjacent to the transcriptionally active downstream constant region, and the intervening DNA is deleted. An enzyme called activation-induced deaminase (AID), which is induced by CD40 signals, plays a key role in this process. AID converts cytosines in the transcribed switch region DNA to uracil (U). The sequential action of other enzymes results in the removal of the U’s and the creation of nicks in the DNA. Such a process on both strands leads to double-stranded DNA breaks. When double-stranded DNA breaks in two switch regions are brought together and repaired, the intervening DNA is removed, and the rearranged VDJ exon that was originally close to Cμ may now be brought immediately upstream of the constant region of a different isotype (e.g., IgG, IgA, IgE). The result is that the B cell begins to produce a new heavy-chain isotype (determined by the C region of the antibody) with the same specificity as that of the original B cell, because specificity is determined by the sequence of the VDJ exon, which is not altered.

Fig2. Mechanism of immunoglobulin heavy-chain iso type switching. In an immunoglobulin (Ig)M-producing B cell, the rearranged VDJ encoding the V region is adjacent to the μ constant region genes (Cμ). Signals from helper T cells (CD40 ligand and cytokines) may induce recombination of switch (S) regions such that the rearranged VDJ DNA is moved close to a C gene downstream of Cμ, which are Cγ genes in the example shown. The enzyme activation-induced deaminase (AID), which is induced in the B cells by signals from Tfh cells, alters nucleotides in the switch regions so that they can be cleaved by other enzymes and joined to downstream switch regions. Subsequently, when the heavy-chain gene is transcribed, the VDJ exon is spliced onto the exons of the downstream C gene, producing a heavy chain with a new constant region and thus a new class of Ig. Note that although the C region changes, the VDJ region, and thus the specificity of the antibody, is preserved. (Each C region gene consists of multiple exons, but only one is shown for simplicity.)

Cytokines produced by follicular helper T cells determine which heavy-chain isotype is produced (see Fig. 1). The production of opsonizing IgG antibodies, which bind to phagocyte Fc receptors, is stimulated by IL-10 and other cytokines in humans and mainly by IFN-γ in mice. In antibody responses, these cytokines are produced by Tfh cells. The IgG antibodies that are produced opsonize microbes and promote their phagocytosis and intracellular killing. By contrast, switching to the IgE class is stimulated by IL-4 produced by Tfh cells. IgE functions to elim inate helminths, acting in concert with eosinophils, which are activated by another Th2 cytokine, IL-5. Predictably, helminths induce strong Th2 and related Tfh cell responses. Thus, the nature of the helper T cell response to a microbe guides the subsequent anti body response, making it optimal for combatting that microbe. These are excellent examples of how different components of the immune system are regulated coordinately and function together in defense against different types of microbes and how helper T cells may function as the master controllers of immune responses.

The antibody isotype produced is also influenced by the site of immune responses. For example, IgA antibody is the major isotype produced in mucosal lymphoid tissues, probably because cytokines such as transforming growth factor (TGF)–β that promote switching to IgA are abundant in these tissues. IgA is the principal antibody isotype that can be actively secreted through mucosal epithelia. B-1 cells also appear to be important sources of IgA antibody in mucosal tis sues, especially against nonprotein antigens.