The first therapeutic antibodies were obtained by immunizing large animal (e.g., horses) against the targeted antigen (e.g., diphtheria toxin), resulting in the production in the serum of large, polyclonal antibodies specifically recognizing multiple epitopes of the antigen. Then the serum of these animals was used to treat patients (e.g., diphtheria). These polyclonal antibodies were responsible for many adverse events in patients, mainly because of their allogeneic nature that could be recognized and rejected by the host immune system.

Köhler and Milstein described in 1975 the hybridoma technique which allows to generate antigen-specific monoclonal antibodies [5]. This technique consisted in vaccinating rodents (mice, hamsters, etc.) with a specific antigen and subsequently collects the B cells from the spleen of the immunized animals. These B cells were then fused with an antibody nonproducing plasma cell cancer cell line (myeloma). These fusions between a primary B-lymphocyte and a myeloma cell are called “hybridoma.” The mixture of hybridoma cells, made of multiple different primary B-cell clones, each secreting one specific antibody, could be separated into individual single cell culture wells. Those wells can then be screened to identify the ones which contain the hybridoma secreting the desired antibody. Hybridomas can be expanded in vitro in culture, and antibodies can be concentrated and purified from the culture supernatant.

Since the hybridoma technique has been described, numerous other techniques have been developed, generally with the aim of not only improving the yield of monoclonal antibodies production but also reducing the immunogenicity of monoclonal antibodies [6]. Indeed, rodent-derived monoclonal antibodies can be recognized as foreign by the human immune system which can generate human anti-mouse antibodies (HAMA). Patients developing high levels of HAMAs can increase the clearance rate of the monoclonal antibody from their blood-reducing exposure/efficacy. HAMAs can also be responsible for serious anaphylactic or anaphylactoid adverse reactions. Therefore, many strategies have been developed to reduce the immunogenicity of rodent monoclonal antibodies, which have allowed to generate chimeric murine/human monoclonal antibodies or humanized monoclonal antibodies and eventually fully human monoclonal antibodies. The degree of humanization of monoclonal antibodies is specified in their name (Fig. 18.2). Chimeric monoclonal antibodies contain the rodent heavy and light variable chains from the Fab component, thereby retaining the antigen-binding specificity of the initial rodent monoclonal antibody. These variable parts are genetically fused to the human immunoglobulin constant regions (constant Fab and constant Fc regions). This chimerization was a way of preserving the specificity and affinity of the antibody while reducing its immunogenicity in immunocompetent patients. Humanized monoclonal antibodies consist in grafting human framework regions into the variable regions in order to only retain the murine complementarity-determining regions (CDRs) which have the antigen-binding ability. Fully human monoclonal antibodies have been generated, thanks to transgenic mice bearing human immunoglobulin genes. These mice can therefore produce fully human monoclonal antibodies upon immunization. Fully human monoclonal antibodies have significantly reduced immunogenicity and therefore generate low levels of human antihuman antibodies (HAHAs). The low titers of HAHAs usually do not impact significantly the pharmacokinetics of the human monoclonal antibodies and generate low levels of infusion-related reactions (<5% of patients) [7].