Immunity, humoral regulation

One way in which the immune system responds to pathogens is by producing soluble proteins called antibodies. This is known as the humoral response and involves the activation of a special set of cells known as the B lymphocytes , because they originate in the bone marrow. The humoral immune response helps in the control and removal of pathogens such as bacteria , viruses , fungi , and parasites before they enter host cells. The antibodies produced by the B cells are the mediators of this response.

The antibodies form a family of plasma proteins referred to as immunoglobulins . They perform two major functions. One function of an antibody is to bind specifically to the molecules of the foreign agent that triggered the immune response. A second antibody function is to attract other cells and molecules to destroy the pathogen after the antibody molecule is bound to it.

When a foreign agent enters the body, it is engulfed by the antigen-presenting cells, or the B cells. The B cell that has a receptor (surface immunoglobulin) on its membrane that corresponds to the shape of the antigen binds to it and engulfs it. Within the B cell, the antigen-antibody pair is partially digested, bound to a special class of proteins called MHC-II, and then displayed on the surface of the B cell. The helper T cells recognize the pathogen bound to the MHC-II protein as foreign and becomes activated.

These stimulated T cells then release certain chemicals known as cytokines (or lymphokines) that act upon the primed B cells (B cells that have already seen the antigen). The B cells are induced to proliferate and produce several identical cells capable of producing the same antibody. The cytokines also signal the B cells to mature into antibody producing cells. The activated B cells first develop into lymphoblasts and then become plasma cells, which are essentially antibody producing factories. A subclass of B cells does not differentiate into plasma cells. Instead, they become memory cells that are capable of producing antibodies at a low rate. These cells remain in the immune system for a long time, so that the body can respond quickly if it encounters the same antigen again.

The antibody destroys the pathogen in three different ways. In neutralization, the antibodies bind to the bacteria or toxin and prevent it from binding and gaining entry to a host cell. Neutralization leads to a second process called opsonization . Once the antibody is bound to the pathogen, certain other cells called macrophages engulf these cells and destroy them. This process is called phagocytosis . Alternately, the immunoglobulin IgM or IgG can bind to the surface of the pathogen and activate a class of serum proteins called the complement , which can cause lysis of the cells bearing that particular antigen.

In the humoral immune response, each B cell produces a distinct antibody molecule. There are over a million different B lymphocytes in each individual, which are capable of recognizing a corresponding million different antigens. Since each antibody molecule is composed of two different proteins (the light chain and the heavy chain), it can bind two different antigens at the same time.

See also Antibody and antigen; Antibody-antigen, biochemical and molecular reactions; Antibody formation and kinetics; Immune system; Immunity, active, passive and delayed; Immunity, cell mediated