Helper T cells or Th cells have the CD4 molecule in them. They secrete cytokines (chemical messengers). There are two types of Th cells. The Th1 cell makes cytokines that regulate the immune response by acting on other T cells and macrophages. The Th2 cell also secretes cytokines but these act on B cells so they can turn into plasma cells that make antibodies. T helper cells are necessary for most situations where an antigen goes on to make antibodies. Cytotoxic T cells or Tc cells do the actual killing of infected cells in the same way as NK cells do. They either express the Fas ligand (that binds to Fas receptors on infected cells) or make perforins and granzymes to cause apoptosis from within the cell. Regulatory T cells or Treg cells are also referred to as suppressor T cells. These have several molecules on their cell surface. The first is the CD4 molecule (making them similar to helper cells). The others are called CD25 and FOXP3. It is believed that they suppress clonal expansion so that the immune process does not go unchecked. It is a cell type that isn’t completely understood.
ANTIBODIES AND B CELLS Antibodies are also referred to as immunoglobulins. They are made by B cells in response to a specific antigen. They are actually receptors on B cells that get secreted into the blood stream in most cases. Some are not secreted but are called surface immunoglobulins on the B cell. There are five kinds of antibodies, called IgM, IgD, IgG, IgA, and IgE. Each antibody type acts slightly differently in the immune system that are effective in killing pathogens. Remember that B cells differentiate and mature mainly in the bone marrow. They undergo central tolerance, which is the destruction of any B cells that recognize self-antigens. Clonal deletion happens to immature B cells that bind too strongly to self-antigens (preventing the problem of self-antibodies). It causes these cells to die off. In clonal anergy, the same thing happens to the B cell; however, the B cell isn’t deleted but is unable to function. Remember, too, that mature B cells do not have the ability to make antibodies until they get the go-ahead from Th2 (helper T cells). If the B cell does not get a signal from the Th2 cell, it gets a signal to die and undergoes apoptosis. This is called peripheral tolerance. Once the B 264
