Immune System

All vertebrates have an immune system. The defense system found among invertebrates is much more primitive often relying primarily on phagocytic (engulfing and digesting) cells. Plants have no immune system.

Immunology, the study of the immune system, grew out the observation that people that recover from certain infections are "immune" from a recurrence of the infection. They rarely develop the same disease again.

Although the immune system evolved to protect vertebrates from infection by microorganisms and larger parasites, most of what we know about immunity has come from the studies of responses of laboratory animals to injections of noninfectious materials such as foreign proteins and polysaccharides. Almost any macromolecule and some rather small molecules are capable of eliciting an immune response.

The thing that elicits an immune response is referred to as an antigen: antibody generator. The thing elicited is an antibody and as remarkable as it seems, the immune system can distinguish between antigens that are very similar: those differing by as little as a single amino acid.

There are two broad classes of immune responses:

• antibody responses
• cell-mediated immune responses

Antibody responses involve the production of antibodies, proteins called immunoglobulins. These circulate in the blood stream, diffuse into tissues and are able to bind specifically to the foreign antigen that induced them.

Cell-mediated immune responses involve the production of specialized cells that react with foreign antigens on the surface of other host cells. The reacting cell can kill a virus-infected host cell that has viral proteins on its surface, thereby eliminating the infected cell before the virus has replicated.

The cells responsible for immune specificity are a class of white blood cells known as lymphocytes. They are found in large numbers in circulating blood as well as in the lymphatic vessels and lymphoid organs (thymus, spleen, appendix and lymph nodes). There are about 2 x 10¹² lymphocytes in the human body which gives the immune system a tissue mass comparable in size to that of the liver.

In experiments in the 1960s, it was discovered that there are two major classes of lymphocytes: T cells and B cells.

T cells which develop in the thymus and are responsible for cell-mediated immunity and B cells which develop in the bone marrow or fetal liver and are responsible for antibodies. Both derive ultimately from the stem cells which give rise to all of the blood cells. These stem cells are located primarily in hemopoietic (blood forming) tissues, the liver in fetuses and the bone marrow in adults.

In vertebrates, T cells develop in the thymus from precursor cells that migrated there from the hemopoietic tissues. The thymus is referred to as a primary lymphoid organ.

Most lymphocytes die soon after they develop in a primary lymphoid organ. Others migrate to a secondary lymphoid organ: mainly the lymph nodes, spleen and gut associated lymphoid tissues (appendix, tonsils, adenoids and Peyer's patches). It is mainly in these secondary lymphoid tissues that T cells and B cells react with foreign antigens.

T cells and B cells become morphologically distinguishable only after they have been stimulated by antigen. An active B cell develops into an antibody secreting cell, the most mature of which is referred to as a plasma cell. Plasma cells are large cells and are rich in rough ER.

Activated T cells do not have much ER and do not secrete antibodies.

One can also differentiate between T and B cells by noting differences in their plasma membrane glycoproteins.

A most remarkable feature of the immune system is that it can respond to millions of different foreign antigens in a highly specific way. How can the immune system produce such a diversity of specific antibodies? How can the immune system produce antibodies to materials simply not found in nature? A puzzle.

For many years, the hypothesis was that antibody molecules were "unfolded proteins" whose final conformation is determined by the antigen around which they fold. This notion had to be abandoned when chemists discovered that the three-dimensional structure of a protein is determined by its amino acid sequence.

The instruction hypothesis was discarded in the 1950s in favor of the clonal selection theory. This is based on the notion that during development, each lymphocyte becomes committed to react with a particular antigen before ever being exposed to it. A cell expresses this commitment in the form of cell surface receptor proteins that specifically "fit" the antigen. Binding of antigen to the cell activates it, causing it to proliferate and mature.

So, a foreign antigen selectively stimulates those cells that express complementary antigen specific receptors and are thus already committed to respond to it. This makes an immune response antigen specific.

The term "clonal" in clonal selection derives from the fact that the immune system is made up of millions of different families or clones of cells each consisting of T or B lymphocytes descended from a common ancestor.

Since each ancestral cell is already committed to make one particular antigen specific receptor protein, all cells in a clone have the same antigen specificity.

The clonal selection theory is a "ready-made" rather than a "made to order" kind of thing. The question of how an animal makes so many different antibodies becomes a problem of genetics then rather than protein chemistry.

There is compelling evidence to support the clonal theory of selection.

Most antigens stimulate MANY different lymphocyte clones. Most antigens have a variety of antigenic determinants or epitopes that stimulate the production of antibodies or T cell responses.

Even an antigen that activates many clones will stimulate only a tiny fraction of the total lymphocyte population. Antigens tend to circulate through secondary lymphoid tissues through which B and T cells continuously circulate. The continuous circulation is important not only to expose B and T cells to antigen; it also exposes lymphocytes to lymphocytes --- interactions between specific lymphocytes are a crucial part of most immune responses.

Lymphocytes have "homing receptors" on them that select for particular secondary lymphoid tissues. When they are activated by antigen, they loose these homing receptors and acquire new ones that guide the activated cells to sites of inflammation.

The immune system, like the nervous system, can remember. This is why, usually one siege of chicken pox confers immunity from subsequent infections.

If you inject an animal with an antigen, its immune response will appear after a lag period of several days, rise rapidly and exponentially and then fall --- a primary immune response.

If some weeks or months or years are allowed to elapse and the animal reinjected with antigen , it produces a secondary immune response that is very different from the primary response. The lag period is shorter and the response is greater and its duration is longer. Consider that when young children are immunized for DPT, diphtheria, pertussis (whooping cough) and tetanus, they receive a series of shots. This is because of the primary and secondary responses.

The failure to respond to self antigens is due to acquired immunological tolerance. Normal mice cannot make an immune response against a blood protein called C5. Take a mutant mouse that is deficient in the gene encoding C5 but otherwise genetically identical. Inject it with C5 --- it makes an immune response to C5. This shows that the immune system is capable of responding to self but learns not to do so.

In some cases, the learning involves eliminating self-reactive lymphocytes although it is not well understood how this is achieved.

Tolerance to self antigens sometimes breaks down, causing T or B cells to react against their own tissue antigens. Myasthenia gravis is an autoimmune disease. Antibodies form against acetylcholine receptors on their own skeletal muscle cells. Lupus erythmatosus is another autoimmune disease. Rheumatoid arthritis is also an autoimmune disease.

Immunological tolerance can be induced:

1. antigens in very high doses
2. in repeated very low doses
3. with an immunosuppressive drug
4. intravenously after the antigen has been chemically coupled to the surface of B lymphocytes.

The molecular mechanisms are not well understood but seems to depend on maturity of the lymphocyte, nature and concentration of antigen and complex interactions between lymphocytes and so-called antigen presenting cells.

Vertebrates rapidly die of infection if they are unable to make antibodies. Synthesized exclusively by B-lymphocytes, antibodies are produced in millions of forms, each with a different amino acid sequence and a different binding site. There are 5 classes of antibodies or Ig (immunoglobulin). Time will not not allow even the beginning of a discussion on what is known about the structure of these proteins.

Let's talk a bit instead about the generation of antibody diversity. It is estimated that even in the absence of antigen stimulation, a mouse can make many millions of different antibody molecules... its pre-immune repertoire.

Antibodies are proteins and proteins are encoded by genes. Antibody diversity therefore poses a special genetic problem. How can an animal make more antibodies than there are genes in its genome? The human genome is thought to contain fewer than 105 genes. This problem is not as formidable as it first appears. Because the variable regions of both the L and H chains contribute to an antigen binding site, an animal with 1000 genes encoding L and 1000 genes encoding H chains could combine their products in 1000 X 1000 different ways to make 106 different antigen binding sites.

Turns out that the immune system has evolved unique genetic mechanisms that enable it to generate an almost unlimited number of different L (light) and H (heavy)chains.

The L and H chains consist of constant and variable regions. The binding site of antibodies is formed by only about 20-30 of the amino acid residues in the variable region of each chain. These are referred to as hypervariable regions. A single C gene segment encodes the C (constant)region of an Ig chain. More than one gene segment encodes each V (variable) region.

Each L chain V region is encoded by a DNA sequence assembled from two gene segments, a long V gene segment and a short J (joining) gene segment. There are 300 or so V gene segments and 4 J segments for the L chain.

Each H chain V region is encoded by a DNA sequence assembled from three gene segments, a V gene segment, a J gene segment and a D (diversity) gene segment. There are 1000 or so V segments, 4 J segments and at least 12 D segments.

Any of the 300 or so V segments can be joined with any of the 4 J segments for the light chain so at least 4 x 300 = 1200 different things can be coded by this pool.

Any of the 1000 or so V segments can be joined with any of the 4 J segments and of at least 12 D segments so at least 1000 x 4 x 12 = 48000 different heavy chains can be coded for.

All of this is referred to as combinatorial diversification. In a mouse, it is estimated that there might be 1000 different V light regions and 50000 V heavy regions. Combining these gives the potential for 5 x 10⁷ different antigen binding sites.

In addition, the joining mechanism itself greatly increases (maybe 1000 fold). In most cases of site specific recombination, DNA joining is precise. In the case of joining of antibody gene segments, the joining is not precise. A variable number of nucleotides are often lost from the ends of the recombining gene segments and in the case of the heavy chain, one or more randomly chosen nucleotides may also be inserted. This random loss or gain of nucleotides at joining sites (called junctional diversification) enormously increases the diversity of V region coding sequences. This is not without cost. Sometimes results in a shift in the reading frame so that a nonfunctional gene will be produced. This commonly occurs.

Complement, so called because it complements and amplifies the action of antibody is the principal means by which antibodies defend vertebrates against most bacterial infections.

Complement consists of a system of serum proteins that can be activated by antigen-antibody complexes to undergo a cascade of proteolytic reactions whose end result is the assembly of membrane attack complexes. These complexes form holes in a microorganism and destroy it.

An additional part of the complement activation process dilates blood vessels and attracts phagocytic cells to sites of infection.

Complement consists of about 20 interacting proteins.

All in all, the immune system is a very interesting one.

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