The Antibody Response
Animals born and reared in the complete absence of contact with any non-self material (not an easy procedure!) have virtually no immunoglobulins in their serum, but as soon as they encounter the normal environment, with its content of microorganisms, their serum immunoglobulin (Ig) rises towards the normal level of 10–20 mg (or about 60 000 000 000 000 000 molecules) per millilitre. This shows that immunoglobulins are produced only as a result of stimulation by foreign antigens, the process being known as the antibody response. In the figure, these events are shown in a section through a stylized lymph node. Antigen is shown entering from the tissues (top left) and antibody being released into the blood (bottom right). The antigen is depicted as a combination of two components, representing the portion, or determinant, recognized by the B cell and against which antibody is eventually made (black circles) and other determinants that interact with T cells and are needed in order for the B cell to be fully triggered (white triangles). These are traditionally known as ‘haptenic’ and ‘carrier’ determinants, respectively. In practice, a virus, bacterium, etc. would carry numerous different haptenic and carrier determinants, whereas small molecules such as toxins may act as haptens only. But even small, well-defined antigenic determinants usually stimulate a heterogeneous population of B cells, each producing antibody of slightly different specificity and affinity.
The main stages of the response are recognition and processing of the antigen (see Fig. 18), selection of the appropriate individual B and T cells (shown larger in the figure), proliferation of these cells to form a clone, and differentiation into the mature functioning state. A prominent feature of all stages is the many interactions between cells, which are mediated mostly by cytokines (white arrows in the figure). There are also a number of regulatory influences whose relative importance is not yet clear. Most of these cell interactions occur in the lymph nodes or spleen, but antibody can be formed wherever there is lymphoid tissue.
In a subsequent response to the same antigen, average affinity tends to be higher, precursor T and B cells more numerous and Ig class more varied. This secondary response is therefore more rapid and effective, and such an individual is described as showing memory to the antigen in question; this, for example, is the aim of most vaccines (see Fig. 41).
AL Afferent lymphatic, via which antigens and antigen-bearing cells enter the lymph node from the tissues (see Fig. 17).
APC Antigen-presenting cell. Before they can trigger lymphocytes, antigens normally require to be presented on the surface of a specialized cell. These cells form a dense network within lymphoid tissue, through which lymphocytes move around searching for antigen. B cells recognize antigen on the surface of follicular dendritic cells (FDC), while T cells interact with interdigitating dendritic cells (IDC) which carry high levels of MHC and costimulatory molecules.
FDC The follicular dendritic cell, specialized for presenting antigen to B lymphocytes in the B-cell follicles. Antigen on the surface of FDC is largely intact, maintaining its native conformation, and is often held in the form of antibody – antigen complexes that can persist for weeks or months.
IDC The interdigitating dendritic cell, specialized for presenting pep- tides to T cells in the T-cell area or paracortex. Some IDC develop directly within lymph node or spleen (‘resident’ dendritic cells) while others take up antigen in non-lymphoid tissues (those in skin epidermis, for example, are known as Langerhans’ cells) by pinocytosis or phagocytosis, process it (see Fig. 18) and then migrate through the afferent lymphatics to the nearest lymph node.
Selection Only a small minority of lymphocytes will recognize and bind to a particular antigen. These lymphocytes are thus ‘selected’ by the antigen. The binding ‘receptor’ is surface Ig in the case of the B cell, and the TCR complex in the case of the T cell, which recognizes both antigen and MHC (see Figs 11 and 12).
Clonal proliferation Once selected, lymphocytes divide repeatedly to form a ‘clone’ of identical cells. The stimuli for B-cell proliferation are a variety of T-cell-derived cytokines and adhesion molecule interactions (see Fig. 12). T-cell proliferation is greatly augmented by another soluble factor (IL-2) made by T cells themselves. (For more information on interleukins see Figs 23 and 24.) The combination of selection by antigen followed by clonal proliferation has given to the whole lymphocyte response the descriptive name clonal selection. As the immune response progresses, B cells with higher and higher specificities are preferentially selected giving rise to affinity maturation.
Differentiation Once they have proliferated, B cells require help from T helper (TH) cells to progress though further steps of differentiation. T-cell helper signals include both direct cell contact via receptors and their ligands (e.g. the interaction of CD40 on the T cell with CD40Ligand on the B cell) and soluble cytokines. An important aspect of differentiation is isotype switching, the ability of a B cell to produce a different class of antibody. Isotype switching is regulated mostly by specific interleukins. IgE production, for example, requires the release of IL-4 by a subset of TH cells known as TH2 cells. Certain large repeating antigens can stimulate B cells without T-cell help; they are called ‘T independent’ and are usually bacterial polysaccharides. In the absence of T cell help (e.g. in certain genetic diseases in which CD40 or CD40L are absent) B cells secrete only IgM, and do not progress to isotype switching, memory cell formation or hypermutation.
Plasma cell In order to make and secrete antibody, endoplasmic reticulum and ribosomes are developed, giving the B cell its basophilic excentric appearance. Plasma cells can release up to 2 000 antibody molecules per second. They stop circulating and are found predominantly in bone marrow or in the medulla region of lymphoid tissue. Most only live for a few days, but a much longer-lived subpopulation of plasma cells may also exist. Plasma cells can be chemically fused to tumor cells to form cellular hybrids. Some of these hybrids retain the tumour property of immortality, while continuing to produce their specific antibody. B-cell ‘hybridomas’ have been used to produce a huge array of monoclonal antibodies, which are now widely used in biology and medicine as molecular tools to isolate or classify molecules and cells. More recently, monoclonal antibodies are being used as drugs to treat cancer and autoimmunity, and potentially a range of other diseases.
EL Efferent lymphatic, via which antibody formed in the medulla reaches the lymph and eventually the blood for distribution to all parts of the body.
Memory cells Instead of differentiating into antibody-producing plasma cells, some B cells persist as memory cells, whose increased number and rapid response underlies the augmented secondary response, essentially a faster and larger version of the primary response, starting out from more of the appropriate B (and TH) cells. Memory B cells differ slightly from their precursors (more surface Ig, more likely to recirculate in the blood) but retain the same specificity for antigen. The generation of memory B cells requires T cells, because T-independent responses do not usually show memory. TH cells also develop into memory cells. Individual memory cells divide slowly (every few months) but do not appear to require antigen restimulation.
Germinal centres These are the major site of long-term antigen storage (primarily as complexes with antibody and complement, see Fig. 20) and of B-cell proliferation (clonal expansion). They are also the site for somatic hypermutation, a process that introduces small random mutations into the DNA sequence coding for the antibody binding site (see Fig. 13). While most of these mutations will decrease the specificity of the antibody for its antigen, some may increase it, and these will be selected for as the immune response continues, resulting in a general increase in antibody affinity (affinity maturation).
Feedback inhibition Antibody itself, particularly IgG, can inhibit its own formation, by binding the antigen and preventing it stimulating B cells. T cells that suppress antibody production have also been described (originally termed TS, they are now more commonly called TREG). Although these cells have still not been fully characterized (see Fig. 22), they act by regulating the TH cell rather than by directly sup- pressing the B cell itself. In practice the single most important element in regulating antibody production is probably removal of the antigen itself.
Networks It was hypothesized by Jerne, and subsequently confirmed, that antibody idiotypes (i.e. the unique portions related to specificity) can themselves act as antigens, and promote both B-cell and T-cell responses against the cells carrying them, so that the immune response progressively damps itself out. This leads to the intriguing concept of a network of anti-idiotype receptors corresponding to all the antigens an animal can respond to – a sort of ‘internal image’ of its external environment. However, the actual role of networks in regulating ordinary antibody responses is not yet clear.