see also NIH lecture

• The immune system recognizes and eliminates antigens, foreign substances or organisms that enter the body.
• Amoeboid, phagocytic cells, the macrophages and the neutrophils (antigen processing cells (APC)) engulf particles and invaders, massing at infection sites and forming pus.
• After APCs phagocytize and process the antigen, they present the antigen to specific helper T cells.  These helper T cells decide whether the antigen is bacterial or viral and then send a signal to the humoral or cellular system, respectively.

• B-cell lymphocytes and T-cell lymphocytes, the most common immune system cells, participate in the immune response, both humoral (involving antibodies or immunoglobins) and cell-mediated (cellular).  When activated, T-cells contain free ribosomes, while B-cells have bound ribosomes (rough endoplasmic reticulum).
• The immune system is widespread.  Central lymphoid tissue includes the bone marrow and thymus, while peripheral lymphoid tissue includes lymph nodes, spleen, adenoids, tonsils and Peyer's patches.

B and T-Lymphocytes and Clonal Selection

• The manner in which lymphocytes identify specific antigens is proposed by the clonal selection theory.  Virgin cells each bear a specific antigen recognition protein.  When one finds an antigen match, the virgin B-cells undergoes mitosis, producing plasma cells and memory cells, while T-cells produce memory cells and several kinds of T-cells.

• In primary immune response, plasma cells produce antibodies immobilizing the antigen bearing cells or molecules.

• Memory cells store antibody-synthesizing instructions, living quite long and producing clones.  T-cells produce memory cells and several other kinds of T-cells.  If the same antigen appears again, a secondary immune response occurs as the clone produces many plasma cells that rapidly release antibodies. The immune response can be brought on artificially by vaccination.

• Antibodies , or immunoglobins, all contain two identical heavy amino acid chains and two identical light chains.  Each chain also has a constant and variable region.

• The latter contains an antigenic determinant, which is specific to each antigen.
• There five types of antibody that can be produced from each clone.  Each immunoglobin (Ig) has a different structure and function.

• Following their reaction, agglutinated antibody-antigen complexes are readily phagocytized.

• Recombinant DNA techniques now make it possible to produce clones of specific antibody producing lymphocytes.  The B-cell is fused with a tumor cell forming hardier hybridomas, and when cloned, they produce monoclonal antibodies that can be harvested and used to treat disease.

• The question of how B-cells can produce antibodies against all conceivable antigens has produced a number of hypotheses.
• GO TO THE FOLLOWING PAGES
• Antibody Production, Regulation & Diversity by Natural Toxins Research Center at Texas A&M University - Kingsville
• Genetics of the Antibody Loci at University of Texas
• Antigen Receptor Diversity by Kimball
• The Immune System:  Selection by the Enemy by University of Illinois at Urbana-Champaign

The Work of T-Cell Lymphocytes

from P'ng Loke, St. Anne's College, Department of Zoology, University of Oxford

• Cytotoxic T lymphocytes (CTL) play a critical role in host cell-mediated immune responses against viral infections, as well as during the pathology associated with tissue and bone marrow rejection.
• Host cells infected with viruses produce viral proteins, some of which are degraded into small antigenic peptide fragments as part of the antigen processing mechanism.
• Viral peptides will then appear on the cell surface in association with major histocompatibility complex (MHC) class I glycoproteins.
• CTL which recognizes these viral peptide-MHC complexes as being foreign antigens will be triggered to destroy the virally-infected cells.
• Genetically distinct transplanted tissue present a different array of cellular peptides due to the different shape and charge of their MHC groove.
• Recognition of these novel antigen-MHC complexes will also trigger host CTL to destroy transplanted tissues.
• The interaction between CTL and target cells involve:
• 1) adhesion and recognition,
• 2) delivery of a `lethal hit'
• 3) death of the target cell.

Major Histocompatibility Complex from NIH

•     Molecules that mark a cell as self are encoded by a group of genes that is contained in a sections of a specific chromosome known as the major histocompatibility complex (MHC).
• The prefix "histo" means tissue; the MHC was discovered in the course of tissue transplantation experiments.
• Because MHC genes and the molecules they encode vary widely in the details of their structure from one individual to another (a diversity known as polymorphism), transplants are very likely to be identified as foreign and rejected by the immune system.
• Scientists eventually discovered a more natural role for the MHC: it is essential to the immune defenses.
• MHC markers determine which antigens an individual can respond to, and how strongly.
• Moreover, MHC markers allow immune cells such as B cells, T cells, and macrophages to recognize and communicate with one another.
• One group of proteins encoded by the genes of the MHC are the markers of self that appear on almost all body cells.
• Known as class I MHC antigens, these molecules alert killer T cells to the presence of body cells that have been changed for the worse - infected with a virus or transformed by cancer - and that need to be eliminated.
• A second group of MHC proteins, class II antigens, are found on B cells, macrophages, and other cells responsible for presenting foreign antigen to helper T cells.
• Class II products combine with particles of foreign antigen in a way that showcases the antigen and captures the attention of the helper T cell.
• This focusing of T cell antigen recognition through class I and class II molecules is known as MHC (or histocompatibility) restriction.