Antibodies are glycoprotein molecules with a remarkable ability to recognize and bind to antigens with high affinity and specificity, thus further promoting their inactivation or elimination. They are produced by B lymphocytes, which undergo a complex life cycle that culminates in their differentiation into antibody-secreting plasma cells. There are two kinds of immunoglobulins (Igs) that B cells can produce. One is the membrane-bound Ig on the surface of B cells, called the B cell receptor (BCR). The other is the secreted Ig, known as an antibody, which is produced by plasma cells. The secreted antibodies bind pathogens or their toxic products to engage immune response and initiate elimination mechanisms. There are five major types of antibodies with multiple functions: IgA, IgD, IgE, IgG and IgM. Among them, IgGs are the most abundant type of antibodies, as they constitute approximately 75% of the serum immunoglobulin repertoire.
Fig 1. The different classes of immunoglobulins: IgA, IgG, IgD, IgE, IgM. (Batra, 2016)
Antibodies are protagonists of adaptive immunity with key roles in immune defense and immune regulation. Lymphocytes allow responses against a vast array of antigens from various pathogens, such as bacteria, viruses, and other diseases, and confer the important feature of immunological memory. When a foreign antigen enters the body and activates B cells by binding to the BCR, a large clone of identical cells is produced and these cells finally differentiate into effector cells, called plasma cells. These cells can secrete millions of antibodies into the bloodstream and lymphatic system. Most lymphocytes in the immune response will eventually die. However, a significant number of activated antigen-specific cells persist after the elimination of antigen. These cells are known as memory B cells and form the basis of immunological memory. The antigen introduced encounters little specific antibody in the serum at the first time. When the same antigens invade the immune system again, a very rapid and intense antibody secondary response to the antigens occurs, illustrating immunological memory.
Antibodies can inhibit pathogens in the presence or the absence of effector cells or effector molecules such as complements. The biological function of antibodies is to recognize and bind to pathogens and their toxic products to facilitate their removal from the body. Recognition and effector functions are structurally separated in the antibody molecule, variable region-mediated molecular recognition, and constant region-mediated neutralization of viruses and destruction of mark pathogens. The antigen: antibody complex is eventually scavenged and degraded by macrophages. Examples of antibody functions include neutralization of infectivity, phagocytosis, antibody-dependent cellular cytotoxicity (ADCC), and complement-mediated lysis of pathogens or infected cells.
Fig 2. Canonical biological functions of antibodies. (Dimitrov, 2020)
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