17.3 Achieving immunocompetence

• Introduction
  
• Cell-mediated immunity
• Humoral immunity
• Development of the immunologic tolerance
  • Positive selection
  • Negative selection

Cell-mediated immunity

There are two main classes of T-lymphocytes:

• Cytotoxic T cells
• Helper T cells

Cytotoxic T cells recognize cells that are infected with a virus or other intracellular microorganism. Such foreign proteins, which invade a cell as an infection, become partially intracellular dismantled and finally - with the help of MHC-proteins - transported to the cell surface where they are presented to the cytotoxic T cells. These kill the host cell in a direct way in that they trigger a programmed cell death. The helper T cells, on the other hand, stimulate the immune reaction of other cells; for example, they help in the activation of macrophages and B cells.

Both types of T cells express antibody-like receptors on the plasma membrane. These are coded by genes that are put together out of many gene segments during the maturation phase of the T cells in the thymus (see also More info: Interactive diagram, 40 kB). Such receptors recognize antigen fragments that are presented on the host cell's surface in connection with the MHC molecules (major histocompatibility-complex). (see: Development of immunologic tolerance)

Humoral immunity

The circulating antibodies, which are also termed immunoglobulins - abbreviated as Ig - with various classes (Ig A, D, E, G and M, with diverse subclasses), are responsible for humoral immunity. They are produced exclusively by B-lymphocytes. Antibodies constantly and efficaciously defend our bodies in that they inactivate viruses or bacterial toxins and launch the complement-system (see below) or set the various types of white blood cells at phagocytosing microorganisms and even larger parasites. Their absolutely unique characteristic is that they have abundant various forms, of which each possesses a different amino acid sequence and a completely individual binding site for antigens. Immunoglobulins are one of the main blood components, making up roughly 30% of all plasma proteins.