Acquired immunity: antibodies and killer cells at work
If the second line fails to get rid of the invader, the body uses a third line: lymphocytes carry out specific defences called the immune response. It works in two linked ways. Humoral (antibody-mediated) immunity defends against pathogens and toxins in the blood plasma and lymph by producing antibodies. Each B cell has receptors that fit one particular antigen. A B cell that meets its antigen binds it, takes it in and breaks it down, and shows pieces of it on its surface attached to MHC protein. At the same time macrophages engulf the pathogen, break it into fragments with enzymes from their lysosomes, and show the fragments on their surface joined to MHC protein. A helper T cell recognizes this antigen-MHC complex on the macrophage and is activated; it releases interleukins, which activate the B cell. The activated B cell divides and becomes plasma cells, which make large amounts of antibodies that travel through the lymph into the blood, and memory cells, which stay in the blood for a very long time (the book says 20 to 30 years). The antibodies join the antigens on the pathogens, which makes macrophages swallow them, and this goes on for days or weeks. Antibodies are large molecules and cannot reach a virus inside a cell. Such cells are dealt with by cell-mediated immunity, which depends on T cells. The activated helper T cell divides into a clone of active helper T cells and memory helper T cells, and releases interleukins and cytokines. These call more macrophages, stimulate other T cells and B cells, and activate natural killer cells. Cytotoxic T cells recognize transplanted tissue, cells infected by microbes and cancer cells by their receptors, and destroy them, either by punching holes with perforin or by releasing lymphotoxins that damage the nucleus.
| Humoral immunity | Cell-mediated immunity | |
|---|---|---|
| Deals with | pathogens and toxins in plasma and lymph | cells infected by viruses or microbes, transplanted tissue, cancer cells |
| Main cells | B cells, plasma cells, helper T cells | helper T cells, cytotoxic T cells, natural killer cells |
| Weapon | antibodies | perforin and lymphotoxins; cytokines call other cells |
| Memory | memory B cells | memory helper T cells |
Role play, with a grown-up. Make cards for macrophage, helper T cell, B cell, plasma cell, cytotoxic T cell and a virus-infected cell. Act out the humoral steps first, passing a card that says “antigen” from the macrophage to the helper T cell, then “interleukin” to the B cell. Then act the cell-mediated steps, ending with the cytotoxic T cell reaching the infected cell. Say what would go wrong if the antibodies had to reach a virus inside a cell.