27.3 Adaptive immunity  (Page 2/6)

Humoral immune response

As mentioned, an antigen is a molecule that stimulates a response in the immune system. B cells participate in a chemical response to new antigens by producing specific antibodies that circulate throughout the body and bind with the antigen whenever it is encountered. This is known as the humoral immune response , because the active molecule is secreted into the body fluids, or "humours". As discussed, during maturation of B cells, a set of highly specific B cells are produced that have many antigen receptor molecules in their membrane ( [link] ).

Each B cell has only one kind of antigen receptor, which makes every B cell different. Once the B cells mature in the bone marrow, they migrate to lymph nodes or other lymphatic organs. When a B cell encounters the antigen that binds to its receptor, the antigen molecule is brought into the cell by endocytosis. It is digested in the lysosomes, and fragments of the foreign molecule are then displayed on the surface of the B cell. These displayed molecules can activate other cells as part of the immune response. When this process is complete, the B cell is sensitized. In most cases, the sensitized B cell must then encounter a specific kind of T cell, called a helper T cell, before it is activated. The helper T cell must already have been activated through an encounter with the antigen (discussed below).

The helper T cell binds to the displayed antigen fragments on the B cell, and is activated to release cytokines that induce the B cell to divide rapidly. This generates thousands of identical (clonal) B cells. These daughter cells have two possible fates: they can become either plasma cells or memory B cells. The plasma cells produce and secrete large quantities of antibody molecules, up to 100 million molecules per hour, . An antibody , also known as an immunoglobulin (Ig), is a protein that is produced by plasma cells after stimulation by an antigen. Antibodies are the agents of humoral immunity. Antibodies occur in the blood, in gastric and mucus secretions, and in breast milk. Antibodies in these bodily fluids can bind pathogens and mark them for destruction by phagocytes before they can infect cells. The memory B cells become quiescent, and only become reactivated after another later encounter with the antigen. This can be caused by a reinfection by the same bacteria or virus, and activation of the memory cells again results in a new population of antibody-producing plasma cells to fight the re-infection.

These antibodies circulate in the blood stream and lymphatic system and bind with the antigen whenever it is encountered. The binding can fight infection in several ways. Antibodies can bind to viruses or bacteria and interfere with the chemical interactions required for them to infect or bind to other cells. The antibodies may create bridges between different particles containing antigenic sites clumping them all together and preventing their proper functioning. The antigen-antibody complex stimulates the complement system described previously, destroying the cell bearing the antigen. Phagocytic cells, such as those already described, are attracted by the antigen-antibody complexes, and phagocytosis is enhanced when the complexes are present. Finally, antibodies stimulate inflammation, and their presence in mucus and on the skin prevents pathogen attack.