B lymphocytes that recognize antigens proliferate and differentiate into plasma cells that secrete different classes of antibodies with distinct functions. Each clone of B cells expresses a cell surface antigen receptor, which is a membrane-bound form of antibody with a unique antigen specificity. Humoral immune responses may be specific for many different types of antigens, including proteins, polysaccharides, lipids, and small molecules. The response of B cells to protein antigens requires activating signals (help) from CD4+ T cells (which is the historical rea son for calling these T cells helper cells). B cells can respond to nonprotein antigens without the participation of helper T cells. Each plasma cell secretes antibodies that have the same antigen binding site as the B cell surface antigen receptor that first recognized the antigen. Polysaccharides and lipids stimulate secretion of antibodies mainly of the class (isotype) called immunoglobulin M (IgM). Protein antigens also induce the production of IgM antibodies in the early stages of an immune response but may later induce antibodies of different classes (IgG, IgA, and IgE) from a single clone of B cells, a process called heavy-chain class (or isotype) switching. These different antibody classes serve dis tinct functions, mentioned later. Helper T cells also stimulate the production of antibodies with increased affinity for the antigen. This process, called affinity maturation, improves the effective ness of the humoral immune response.

The humoral immune response combats microbes in many ways. Antibodies bind to microbes and prevent them from infecting cells, thus neutralizing the microbes. Antibody mediated neutralization is the only mechanism of adaptive immunity that stops an infection before it is established; this is why eliciting the production of potent neutralizing antibodies is a key goal of vaccination. IgG antibodies coat microbes and target them for phagocytosis because phagocytes (neutrophils and macrophages) express receptors for parts of IgG molecules and are able to bind and ingest microbes coated with these antibodies. IgG and IgM activate the complement system, and complement products promote phagocytosis and destruction of microbes. IgA is secreted from mucosal epithelia and neutralizes microbes in the lumens of mucosal tis sues, such as the respiratory and gastrointestinal tracts, thus preventing inhaled and ingested microbes from infecting the host. Maternal IgG is actively transported across the placenta and protects the newborn until the baby’s immune system becomes mature. Most IgG antibodies have plasma half-lives in the circulation of approximately 3 weeks, whereas other classes of antibodies have half-lives of just a few days. Some antibody-secreting plasma cells migrate to the bone marrow or mucosal tissues and live for years, continuing to produce low levels of antibodies. The antibodies that are secreted by these long-lived plasma cells provide immediate protection if the microbe returns to infect the individual. Additional protection is provided by memory B cells that are activated by the microbe and rapidly differentiate to generate large numbers of plasma cells.

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مواضيع ذات صلة

Major Features of Cell-Mediated Immunity

Initiation and Development of Adaptive Immune Responses

Chemokines and Chemokine Receptors

Overview of Humoral and Cell-Mediated Immunity

Fundamental Properties of Adaptive Immune Responses

Colitis, Autoimmunity, and Primary Immunodeficiencies

Hemophagocytic lymphohistiocytosis

Allergic Rhinitis

Mechanisms of Immune Dysregulation in Autoimmune Disease

Chimeric Antigen Receptor T Cells

Checkpoint Inhibition Therapy for Cancer

Mechanisms of Regulation of T-Cell Activation