Specific Defenses Against Microbial Infection

I. Specific immunity involves recognition of pathogens or other foreign substances by certain cells of our body. Included in the cells that act in a specific immune response are the B-cells (humoral immunity) and the T-cells (cellular immunity).

A. Humoral immunity involves lymphocytes known as B-cells. The B-cells are found in the blood and lymphatic fluids but are produced and mature within the bone marrow. They are responsible for the production of proteins known as antibodies.

B.  Cellular immunity involves cells known as T-cells that are produced in the bone marrow but mature in the thymus gland. These cells are involved in recognizing and destroying virus infected cells. T-cells are also responsible for differentiating between cells that are normal from those that are cancerous or in some way different. The properly functioning cellular immune system should recognize a potentially cancerous cell and destroy it before it can grow into a tumor or spread to other parts of the body.

II. Introduction of certain foreign proteins into the body causes the body to mount an immune response. The foreign proteins that stimulate the immune response are known as immunogens or antigens. An immunogen is functionally defined as anything that can elicit an immune response whereas a antigen is anything that is acted upon by that immune response. For our purposes immunogens and antigens are the same thing. Receptors on the surface of the lymphocyte will recognize a specific foreign protein and will stimulate a response by that cell to the foreign protein.

A.  Proteins and glycoproteins on the surface of bacteria and viruses are antigenic. Only a small region of these proteins are recognized by a receptor on the surface of a specific cell of the immune system. These portions of an antigen that are recognized are known as epitopes or determinants.   An epitope has a unique three dimensional shape and  charge distribution that is recognized by particular cells of the immune system.   Polysaccharides, like those that make up the capsule of many bacteria, can also serve as antigens but often are poorly recognized by the B and T cells.

B. For a substance to be recognized by the immune system it must have a certain size (>10kd). Substances smaller than this threshold must be coupled with large proteins or carbohydrates before they will stimulate an immune response. Most drugs (ex. aspirin, ibuprofen, etc.) are smaller than 10kd thus they fail to stimulate the immune system.

III. When we are born our immune systems are said to be naive for all pathogens. In this state we have billions of lymphocytes of all varieties. Each lymphocyte has a unique receptor that will recognize a slightly different epitope. (A particular lymphocyte only recognizes a single epitope.)

A. One thing that all cells of the immune system have in common is that when they come in contact with the epitope that they are able to recognize they undergo rapid cell division. Every cell that results from the division of the original cell will  recognizes the exact same epitope. This is referred to as clonal expansion. Understanding clonal expansion and the role it plays in developing protective immunity is crucial to understanding the immune system.

B. While you are in the uterus and shortly thereafter the immune system learns to recognize those proteins that are "self". During this period any lymphocyte that recognizes a protein found on the surface of your cells is destroyed. This is referred to as clonal deletion. For obvious reasons, it is important the that the immune system not attack normal tissues within the body. Throughout your life lymphocytes will be generated that will recognize self-proteins. These cells must also be eliminated.  In healthy individuals, deletion of these lymphocytes that recognize your own body occurs in the thymus.  If it does not occur the individual will suffer from autoimmunity.  These lymphocytes recognize self-proteins and attack the cells displaying these proteins or generate antibodies that bind to these proteins leading to inflammatory changes in the affected tissue.  

IV. In the past 20 years an interrelationship between certain types of cells that were once assumed to act completely independently has begun to be understood. Two aspects of this interrelationship will be discussed in this class.

A. Recognition of an antigen by T cells and by some B cells requires that that antigen be “presented” by another cell in a process known as antigen presentation.  Cells that are capable of antigen presentation are referred to as antigen presenting cells (APC’s). 

1.  Many different cells can carry out this process including macrophages, Langerhans cells (found in the skin), dendritic cells and Kupfer cells.  Antigen presentation involves the phagocytosis of bacteria and other foreign substances from the interstitial spaces within our tissues or in the lymph as it flows through a lymph node. Once the foreign substances have been phagocytized they are broken down and small pieces of the proteins from these substances are returned to the surface of the APC bound to a protein known as the major histocompatibility antigens (MHC) (in humans these proteins are also referred to as human lymphocyte antigens or HLA).  There are two major subdivisions of the MHC, MHC type I and MHC type II.   MHC type II participates in antigen presentation as it is described above.  MHC type I will be discussed shortly. 

2. It is becoming increasingly apparent that the dendritic cell plays the crucial role in the initial recognition of antigens by T-cells (both helper and cytotoxic T cells). The dendritic cell will present antigen using both MHC I and MHC II thus it is able to present antigen to both the helper and cytotoxic T cell in the lymph node.  Once stimulated by this antigen presentation the lymphocytes will leave the lymph node, circulate in the blood stream and migrate from the blood steam into infected tissues. 

3. B lymphocytes also play a role in antigen presentation.  They bind specific antigens and then internalize these antigens.  The antigen is digested and fragments of the antigen are then returned to the B cell surface associated with MHC type II.

B. The role played by certain lymphocytes is mainly one of stimulating other cells of the immune system to become more active. This is accomplished by the release of chemicals known as lymphokines (including interleukins), cytokines and interferons. These chemicals act upon other cells increasing their ability to react to pathogens.

V. Antibodies consist of several proteins attached to one another. The antibody has a constant region that is virtually alike in all antibodies of the same class and another region that is variable.  The constant region is referred to as the Fc region.    The variable region is unique to those antibodies produced by one B-cell population. The antibodies bind to antigens by interactions between the variable region and the epitope of the antigen. Antibodies of the class IgG, IgD and IgE are capable of binding two of the same epitope. IgM is capable of binding 10 identical epitopes per molecule. IgA is capable of binding 4 identical epitopes.

A)  Classes of immunoglobulins

1)  IgG is the most common serum antibody.  It is useful in triggering the complement pathway, agglutinating bacteria and viruses, binding up toxins and opsinizing bacteria.  It is capable of crossing the placenta and thus is transferred to the fetus during gestation.

2)  IgM is found on the surface of mature B cells.  IgM, along with IgD, will serve as the antigen receptor.  When antigen binds to the IgM or IgD molecule on the surface of the B cell, the cell undergo clonal expansion and mature into plasma cells.  Plasma cells secrete IgM into the body fluids.   After several days IgM secretion will be curtailed and the antibody IgG will be secreted. 

3)  IgA is the immunoglobulin found in tears and secretions of the mucus membrane and in colostrums.  IgA is also found in the serum at a relatively low level. 

4)  IgD is found on the surface of the mature B cell.  It, along with membrane bound IgM will serve as the antigen receptor on the B-cell surface.  Binding of antigen to membrane bound IgD or IgM triggers the cell to proliferate (clonal expansion) and mature into a antibody secreting plasma cell.  All antibodies produced by that cell and cells derived from it will have the same epitope binding capacity as the membrane bound IgD or IgM.

5)  IgE is produced and secreted into the body fluids where it binds to the surface of basophils and mast cells.  When the antigen that the IgE is recognized is present and binds to the IgE on the surface of these cells it causes them to degranulate.  This release of histamine results in an allergic response to the antigen.

VI. Because there are billions of B cells each with a slightly different receptor recognizing a slightly different antigen almost every protein has one or many epitopes that can be recognized by a B cell and act as an antigen.

A. Binding of the antigen to the receptor (IgM or IgD) on the surface of the B cell in the presence of certain cytokines results in the clonal expansion of that B cell (remember all the resulting cells will have the same receptor as the original B cell). A portion of these cells remain inactive and are referred to as memory cells.   Certain antigens can be recognized by B cells without the need of an antigen presenting cell while other antigens are only recognized when they are presented by an antigen-presenting cell.

B. Final differentiation occurs to some of the activated B-cells and results in the secretion of immunoglobulins (initially IgM and then IgG, IgA or IgE) with the same structure and antigen binding capabilities as the receptor molecule. These antibody secreting cells are referred to as plasma cells.   This switch is dependent on the cytokines released from T cells and contact between the B cell and the T cell.

C. The antibodies which are produced will bind to the antigen.

1. If the antigen is on the surface of a bacteria, fungal cell or virus this antibody can serve as a marker to allow the macrophages to more easily phagocytize these foreign cells. This is referred to as opsinization.
2. Antibodies can also bind to the antigens on the surface of a bacteria or virus thus covering the surface of that virus or bacteria. This inhibits that pathogens ability to bind to and invade tissues and/or cells.
3. Antibodies can also trigger the complement cascade. This group of serum proteins, once activated, serves to rupture the membranes of bacterial cells.
4. Binding of antibodies to a toxin can serve to neutralize that toxin. Often treatment for exposure to a toxin is a shot of antibodies that will recognize that toxin and bind to it. The tetanus vaccine is a toxoid. It resembles the toxin produced by Clostridium tetani and will cause the body to produce antibodies against that toxin.

D. Approximately two weeks after initial exposure to an antigen, antibodies start to show up in the blood in significant numbers, with the initial response peaking about 3 weeks post exposure. This is referred to as the primary response. Upon secondary exposure, the body reacts within days. Antibody levels rapidly increase usually peaking within ten days of exposure. This is referred to as secondary response. This shortened response time minimizes the amount of damage a pathogen can do if the immune system has been exposed to that pathogen previously.

VII. Cellular immunity involves stimulation of T cells.  Several types of T cells are recognized including cytotoxic T cells (also referred to as CTL's or CD8 cells), helper T cells (also referred to as CD4 cells, TH₁  or TH₂ cells) and the T suppressor cells (T_S cells).

A.  Similar to B-cells, T cells are activated by specific binding of the antigen to cell surface receptors. In the case of helper T cells (CD4 cells), antigen must be presented along with MHC type II proteins by antigen presenting cells. On the surface of a helper T cell is an antigen receptor and a protein known as CD 4.  CD 4 seems to play a role in stabilizing the interaction between the antigen receptor and the MHC II protein that is presenting the antigen.

1.  Helper T cell activation results in clonal expansion then the secretion of proteins known as cytokines.  These proteins bind to receptors on the surface of other cells and control the activity of these cells.  In some cases this leads to the proliferation of the cells or increased activity of the target cell.  In other cases the activity of the target cell is suppressed.   It is now clear that there exist two types of helper T cell. 

a. The TH₁ cell secretes cytokines that increase the activity of macrophages and cytotoxic T cells.  When activated these cells increase the inflammatory reaction to a pathogen or injury and suppress the activity of B cells.  The activation of macrophages and other inflammatory cells results in a phenomenon known as delayed hypersensitivity.

b. The TH₂ cell secretes cytokines that increase the response of the B cells and  suppress the activity of the inflammatory cells.

B. Cytotoxic T cells (CTL's) recognize antigen on the surface of cells expressing MHC type I proteins. The antigen is presented on the surface of the cell in a protein known as MHC type I.  The CTL has an antigen receptor and a protein known as CD 8 on its surface. CD 8 seems to stabilize the interaction between the antigen receptor and the MHC type I protein that is presenting the antigen. 

1. Most cells in the body express MHC I proteins on their surfaces. Every cell in the body is constantly making proteins, a sample of these proteins are processed by a structure known as the proteasome.  This cytoplasmic structure chops up the proteins and ships them to the rough endoplasmic reticulum where they are loaded into MHC I molecules.  The MHC I - antigen complexes are moved to the cell's surface. Thus if a cell begins to produce a protein that is abnormal, it will show up on the surface of the cell coupled with the MHC type I protein. 

2. CTL's carryout  surveillance over most of the cells of our body  guarding against alterations in these cells that could indicate virus infection or transformation to a tumor producing cell. When the cytotoxic T cell recognizes an antigen on the surface of a cell it triggers apoptosis (cellular suicide) in that cell by releasing perforins and granzyme. Perforin punches holes in the plasma membrane of the cell allowing granzyme to entry the cytoplasm and activate enzymatic pathways that lead to apoptosis.  In this manner, the CTL destroys  virus infected cells and tumor cells. 

3.  CTL's also increase inflammatory changes in a tissue by secretion of certain cytokines.

4.  The initial stimulation of the CTL is in the lymph node and requires antigen presentation by dendritic cells. Once stimulated, CTL's undergo clonal expansion, leave the lymph node and enter tissues searching for cells that are displaying the antigen they recognize.

C. Natural killer cells (NK cells) are a type of lymphocyte that lacks the CD 4 and the CD 8 cell surface molecules.  NK cells help the body destroy tumor cells and pathogen infected cells in a manner that is similar to the cell destruction that CTL's cause. NK cells differ from CTL's in the mechanisms used to identify abnormal cells within the body. 

1. When cells are infected with a pathogen or have become cancerous, changes in the abnormal cell's surface often occurs. This change may result in underproduction of MHC I molecules and the increased production of one of two cell surface molecules known as MICA and MICB. Natural killer cells recognize the MICA and MICB molecule on the surface of the abnormal cell and, if they do not bind to an MHC I molecule, the NK cell  will release perforin and granzyme to initiate apoptosis in the abnormal cell. 

2. Infected cells will display on their surface proteins from the pathogen infectng them.  If the body has made antibodies against these foreign proteins, the antibody will bind to the infected cell's surface.  On the surface of the NK cell is a receptor for the Fc portion of an antibody (the back end of the antibody). When NK cells bind to the antibody which is bound to the abnormal cell, NK cells release perforin and granzyme that trigger apoptosis in the abnormal cell. This is referred to as antibody-dependent cellular cytotoxicity (ADCC). 

 IX. Allergies result from the production of the class of antibody known as IgE by B cells. This class of immunoglobulin is produced in the same manner as IgG and has the ability to specifically bind to epitopes on foreign proteins in the same manner as other antibodies, but its constant regions has a site that interacts with the surface of a group of cells known as mast cells. Mast cells are found throughout the body in the dermis and submucosa. These cells bind the IgE once it is produced. If the antigen (referred to as an allergen in this case) that initially triggered the production of IgE is introduced into the body a second, time it binds to IgE which is bound to the mast cell. This results in the release of histamines from the mast cell. Histamine is responsible for the classic signs and symptoms of an allergic reaction.

A. Introduction of allergens into local areas (nasal sinuses, GI tract, etc.) result in localized histamine release. Increased vasodilation and vascular permeability leads to swelling and discharge from mucus membranes. This is referred to as local anaphylaxis.

B. Introduction of a an allergen systemically (injection of an allergen) results in a systemic anaphylaxis. Systemic release of histamines and the consequent vasodilation and increase vascular permeability results in rapid loss of blood pressure and anaphylactic shock.
 

Want to know more?  Here are some good links!!!

The Grapes of Staph  Immunity   http://www.cat.cc.md.us/courses/bio141/lecguide/unit3/index.html

Medical Microbiology Textbook  Immunity  Immunology Overview
                        http://gsbs.utmb.edu/microbook/ch001a.htm

Roles of Antigen-Presenting Cells in the Immune Responses against Exogenous Immunogens

http://www.msu.ac.th/bio-dept/Immuno.htm

This site has good explanation of many immunological terms and concepts!!!

Cytokines Online Pathfinder Encyclopedia   http://www.copewithcytokines.de/  

This is one of the better references for a bewildering area of immunology.  I highly recommend this site if you want to begin to understand the complexities of cellular interactions that control the immune system.