Blog

Immunology ~ Medical student education

 

Immunology: A simplified approach.

Aims:

  • Explain the concepts of  immunological surveillance
  • Discuss the basic tools used by the inflammatory cells and
  • Discuss hypersensitivity reactions to explain the different ways the immune response can work.

Discussion:

General

The immune system has been with us since we were unicellular organisms. It has developed for two main reasons both of which are important:

  • To fight infections (external surveillance) and
  • To destroy mutations of self (internal surveillance).

One can consider the immune system to be part of the major defence mechanisms.  It is analogous to how countries set up their defence systems in a multifactorial manner; the FBI monitors internal problems, the CIA monitors external situations, the State Department monitors foreign countries, the Pentagon, the Armed Forces, the Homeland defence, and the Federal drug agency etc. These tend to operate independently but are meant to work together. All of these have counterparts in the immune response.

The body has multiple methods to repel invaders; camouflage in the form of saprophytic bacteria on the skin, natural physical barriers such as keratin and mucus, secreted antiseptics as in sweat and saliva, rapidly turning over epithelial cells, the non-specific immune response (neutrophils and macrophages) and the specific immune response involving T and B lymphocytes and natural killer cells.

The non-specific cells respond to opsinized foreign material, specifically proteins covered by preformed antibodies or complement. This occurs because non-specific cells possess surface receptors which are specific for these opsinized proteins. Neutrophils and macrophages are expendable cells that sacrifice their lives for the benefit of the body (pus is made up of large numbers of sacrificial neutrophils and the tissue debris they produce). Neutrophils in particular are able to enter anoxic tissues (ischaemic areas) because they carry their own respiratory mechanisms (glycolytic pathway).

The key players

The specific type of immune response is based on combination of two receptors:

  • The ‘recognition of self’ (MHC) receptor combined with a receptor recognizing a specific antigenic pattern. The MHC complex can be further subdivided into two big groups; MHC 1 which is expressed on all nucleated cells and MHC 2 which is expressed on antigen presenting cells. The MHC1 is unique to any one individual and truly defines self. MHC 2 drives the immune response. It is this receptor that defines how any one individual will respond to a particular antigenic stimulus.  For example, in any one group of people a cold virus will stimulate different levels of illness, and this is based on your MHC 2 expression.
  • The specific antigen receptor is expressed on B and T lymphocytes, but is different on the two cell types. Remember the antigen is recognized by it’s shape, which will be defined to some extent by the amino acid sequences. However it is the shape and not the amino acid sequence that is recognized. You can shake hands with lots of different people even though hands are of different sizes and shapes.
immune cells

Diagram summarising cells of the immune system (from www.rikenresearch.jp)

The antigen presenting cells include subgroups of macrophages and B lymphocytes. In some situations chronic inflammation can convert epithelial cells into expressing MHC 2 (which may play a role in autoimmune diseases).

  • The antigen presenting macrophages are found throughout the body.  In the skin they are called Langerhan cells. All these cells are capable of motility. They are also found in the lymph nodes in the cortical and paracortical areas. The cells ingest large fragments of antigen, partly break them down and re-express them on the surface in association with MHC, therefore altering and increasing the number of different antigens from the original protein.
  • B lymphocytes recognized repeating antigens such as those seen in bacterial surface membranes and therefore can be stimulated without T helper lymphocytes.  When the cells are stimulated by the correct antigen, they undergo proliferation (blastic) and become either memory circulating B cells or plasma cells. Plasma cells are altered B-cells capable of secreting large amounts of antibody. B lymphocytes express antibody on the surface but do not secrete it.

The T lymphocytes can be broadly divided into two subtypes;

  • T helper cells that have CD4 surface proteins along with MHC2 which are stimulated by a specific antigen on the antigen presenting cells. When a T helper cell is stimulated it undergoes proliferation and secretes cytokines.
  • T cytotoxic cells have CD8 on the surface associated with MHC1 receptors.  These cells kill cells that express a specific antigen in conjunction with MHC1 on the target cells. They kill by either stimulating apoptosis or by directly perforating the cell membrane. Natural killer cells (NK) are similar to CD8 cells, in that they directly kill self.  They respond to alterations in the concentration of MHC expressed on the cell surface.

Polymorph neutrophils contain proteolytic enzymes that destroy membranes. They are recruited into areas of acute inflammation by cytokines and increased blood supply.

Macrophages are part of the phagocytic system.  Different macrophage subtypes are found in different tissues (eg alveolar macrophages in lungs, Kupffer cells in the liver and spleen).

Neutrophils and macrophages have complement and FC receptors on the surface which recognize bound complement components and activated (bound) antibody. An antibody is composed of two light chains and two heavy chains, specific to a particular antigenic shape. It is similar in function to a key. There is a specific component that fits the lock (antigen) termed the Fab area and a long segment which is constant (Fc) which is the part that sticks out for grasping by the macrophages or neutrophils, via their Fc receptors. Remember the Fc component is not antigen specific. When there is cross-linking of these receptors these phagocytic cells are activated. They can either release the enzymes into the surrounding environment or engulf the material and phagocytose. These cells show no immune specificity and will destroy anything that is bound by complement or antibody including normal self.

Eosinophils are similar to neutrophils but are particularly good at killing parasites, because of their cellular contents.

Mast cells are found around blood vessels and are involved in control of blood flow through small vessels such as venules and capillaries. They contain preformed substances and in the process of membrane breakdown, release substances that increase blood flow and constrict smooth muscle.

Antibody is made up of heavy and light chains.

  • The heavy chains define the class ( D,E,G,M)
  • Light chains are either kappa or lambda.

The antibody acts as the B-cell antigen receptor.  Each B-cell has a specific antibody on its surface which is defined by its Fab component and heavy and light chains. So it will carry only one class of heavy chain and one type of light chain. Each class of antibody has subtly different functions.

  • IgM is often poly-valent and is therefore a big molecule and is confined to serum. It is the first antibody to be produced in an inflammatory situation and it’s often more non-specific than the later production of the IgG to the same antigens. Hence acute/active infections can be measured by specific IgM in the serum.
  • IgG (different subclasses) active against bacteria. Being a small molecule, IgG can be found both in the serum and in the extra cellular fluids.
  • IgA can be secreted across epithelial surfaces surfaces and is relatively resistant to digestion. When secreted it is often dimeric (two antibodies joined together). It is found in secretions such as sweat, saliva and gastric juices.
  • IgD is never secreted and is a specific cell surface protein.
  • IgE is used in mast cell activation and is a major trigger in atopic conditions. It is important in parasitic infection (has become less of an issue in Western society). Remember secreted antibodies come from plasma cells and that the B-cell equivalent is the antigen cell surface receptor.

Cytokines are small peptides secreted by stimulated cells that have effects on a wide range of cells. The peptides are rapidly broken down and therefore only work in a small area. There are numerous different types of cytokines and these peptides can have effects on either the cells that secreted them (autocrine), or on other cells, and depending on the effect on the cell each cytokine will have different outcomes. Examples of cytokines include the interferons and interleukins.

The lymph node is the major site for antigenic stimulation.  It is a confined mass of lymphocytes organised into a filtration structure.  There is a capsular sinus which drains the afferent lymphatic fluid.  The lymph, containing substances it has picked up in it’s travels, percolates around the marginal sinus and slowly drips through the lymph node (the sinus is the most common site  for ‘micrometastases’ from a draining malignancy because it is the first point of contact in the lymph node). The lymph node is divided into the cortical and medullary areas.

  • Within the cortex there are the germinal centres which are concentrations of B cells attracted to the antigen presenting cells (dendritic cells). When the B-cells are stimulated, the germinal centres have a central blastic area where the cells undergo proliferation, and a marginal area where they are maturing. Also in the cortex is the paracortical area predominantly composed of T cells surrounded by antigen presenting cells and numerous specialised blood vessels (high endothelial venules) which allow quick transportation of blood cells across the endothelium.
  • The medulla contains mature plasma cells which secrete antibody into the efferent lymphatics. Plasma cells tend not to circulate and are also found in large numbers within bone marrow, where they also have access to blood vessels.

The immune effector system can be simply subdivided into humoral and cell mediated immunity. It is simplistic but explained in methods.

  • The humoral immune system is predominantly used to fight bacteria. This is because the bacteria have long chains of similar antigens on the surface and the antibodies zip up along the antigens, forming a Velcro-type pad for macrophages or neutrophils to engulf.
  • The cell mediated immune system is used predominantly against intracellular pathogens (viruses) and mutations. Large infective organisms are attacked by both systems.
The hypersensitivity reactions:

These are used in to illustrate the different mechanisms of the immune response system. They are over reactions of a normal response and therefore a good way to illustrate these mechanisms.

TYPE 1: ‘A’ for ‘A’sthma or ‘A’naphylaxis

Good examples of this are asthma and anaphylaxis. Someone with asthma is prone to allergic phenomena and people often come from families who have an atopic history.  Asthma takes time to develop because the person has to stimulate the immune response to the recognized antigens.  Antigens vary around the world, the common ones include protein from animals and pollens.  Remember you can also get physical asthma from such things as cold weather and aspirin.

The central cells in asthma and anaphylaxis are mast cells. These are found around small blood vessels. In asthma the allergen is inhaled into the bronchi and crosses the mucosal membrane to reach the mast cells in the sub mucosal vessels.  The cells are primed with antibodies which recognize be allergen.  The mast cell binds the antibody by the Fc component.  When the antigen cross links two or more Fabs the mast cell becomes activated. First it releases preformed vasodilators and bronchoconstrictors (histamine) from the granules with an immediate effect.  As the cell disintegrates it utilises its membranes to form prostaglandins, which also act as vasodilators and bronchconstrictors, and this is why an asthmatic attack can continue for several hours.  As this inflammatory process continues there is tissue damage and often a focal acute inflammatory response is also present. Eosinophils are a feature of allergy because the mast cells release eosinophilic chemo tactic factors.

What are the treatment options for asthma? Desensitisation, where the aim is to convert IgE production to IgG, avoidance of the allergens, stabilisation of mast cells (chromoglycate and dampening down the immune response (steroids , B antagonists).

In summary you have pre-loaded mast cells with specific IgE antibodies, absorbed from plasma cells which, when cross linked, activate the mast cells and results in release of chemicals that increase blood flow and constrict smooth muscle. So this is part of the humoral immune response.

TYPE 2: ‘B’ for Rhesus ‘B’lood reaction

The best example of the type 2 reaction is the Rhesus incompatibility phenomenon; incompatibility occurs when a rhesus negative (Rh-) mother conceives to a rhesus positive (Rh+) father, with a resultant rhesus positive (Rh+) foetus. As the mother has no rhesus antigen, if any foetal red cells enter the maternal circulation, the mother will recognize them as foreign and will make antibodies against the rhesus antigen. This is most likely to occur at the time of birth when there is disruption of the placenta from the uterine wall (but can also occur after miscarriage or abortion). For this reason the first rhesus positive (Rh+) child is spared. However the mother is then ‘primed’ to mount an immunological response (type 2 hypersensitivity response) to any subsequent rhesus positive (Rh+) children; as the Rh+ foetus grows, maternal IgG antibodies to the rhesus protein cross the placenta, bind to the Rhesus antigen on the surface of the foetal red blood cells, activating a humoral response causing cell lysis.

The treatment is preventative – Rhesus -ve mothers receive anti-rhesus antibody at the time of delivery (or at other times the foetal cells may enter the maternal circulation).  The antibody binds to circulating foetal red cells which will then be rapidly destroyed by the reticular endothelial system before the mother can process the ‘foreign’ protein and mount an immune response. This is termed passive immunisation.  Can you give any other examples of passive immunisation?

In summary: Pre-formed antibodies result in cell cytotoxicity termed antibody dependent cell cytotoxicity (ADCC), again part of the humoral response system.  Think of some other examples.

TYPE 3: ‘C’ for immune ‘C’omplex disease.

Immune complex in glomerulusThe best example of this is immune complex disease. ‘Immune complex’ is composed of collections of bound antigen and antibody.  The majority of these occur within the bloodstream, but can occur in extra-cellular spaces. The size of immune complex depends on several factors including the class of antibody and size of the antigen.  The size is both related to physical and electrostatic factors. Immune complexes within the vascular space will activate complement, with all associated consequences. The complement cascade is a complicated system resulting in release of soluble human tactic factors and the production of a final complex that punches holes in cell membranes.  This complex, termed the ‘membrane attack complex’ (think ‘MAC attack’) will punch holes in any membrane it comes in contact with including normal cells.  Often this involves the endothelial cell with resultant thrombosis. Immune complexes are found in many autoimmune diseases (eg SLE) and all these diseases have similar symptoms.  The immune complexes deposit in tissues with a large capillary network, hence kidneys, retina, skin, lung and joints are involved. Large immune complexes in the glomerulus get lodged within the capillariy lumen and therefore damage endothelial cells, setting up an acute inflammatory response with blockage of the capillary by thrombus and inflammatory cells. Clinically the glomerular filtration rate slows so the patient presents with high blood pressure, low urine output which may include red blood cells termed the nephritic syndrome.

If the immune complex is able to penetrate outside the capillary, and away from complement, it has a completely different effect. It acts as a ball and chain destroyer punching holes in whatever membrane it is lodged in. If it is the basement membrane, it has damaged the filter allowing the urine through, resulting in increased urine output and urine that contains proteins, termed the nephrotic syndrome.  The mesangial cells attempt to repair the basement membrane, resulting in ‘membranous glomerular nephritis’. If the immune complexes get further into the mesangium, they stimulate the mesangial cells (part of the macrophages family) resulting in ‘mesangio-proliferation glomerulonephritis’. Sometimes the immune complexes are so small they get to the epithelial cells of Bowman’s space and here they damage the podocytes, which again results in damage to the filtration mechanism and nephrotic syndrome. ‘Crescentic glomerulo nephritis’ is a consequence of severe damage and can occur in multiple immune driven diseases and is a bad clinical finding.

In the joints of patients with rheumatoid arthritis, the immune response creates pannus, which is overgrowth of synovium filled with inflammatory cells and acts as a great proteolytic sponge destroying the surrounding tissues.

In summary, immune complex disease can result from multiple conditions.  The damaged depends on the “size” of the immune complex and where they lodge. It is part of the humoral response system.

TYPE 4: ‘D’ for ‘D’elayed hypersensitivity reaction

A good example of this is tuberculosis. The Mycobacterium is highly resistant to the immune response due to its outer coat (acid fast lipopolysaccharide).  Neutrophils cannot kill it and macrophages can engulf it, but the bacteria is able to switch off the digestive enzymes in the phagosomes. The bacteria is isolated from the external immune response within a cell (macrophage).  It can lie dormant here for many years. Usually the organism is inhaled into the lungs and taken up by alveolar macrophages.  The first exposure may be sub-clinical in that the patient doesn’t even know they have been infected but the organism lies latent in the lung, often in hilar lymph nodes that drain the lung.  The immune response is strong enough to keep it under control but cannot kill the organism. If the strength or integrity of the immune response system wanes (if the patient develops another chronic disease or take steroids) the macrophages no longer control the organism, and the mycobacteria grow and escape from their host cell (macrophage). This allows the organism is to be exposed to the immune response.  As the bacteria is resistant to antibodies, the immune response uses the cell mediated arm to try and destroy the organism.  The cell mediated components include T killer cells and macrophages. Both the cells need to be stimulated by cytokines from T helper cells.  Macrophages can destroy in three ways; (i) as individual activated cells, (ii) as multinucleate giant cells where several cells fuse together to form one massive cell (foreign body type giant cell) or (iii) join together to form a group of activated cells (granuloma). All three of these are seen at the site of infection.  As the organism is resistant to damage, the immune response continues for some time.  The macrophages release large amounts of proteolytic enzymes. This results in significant local tissue damage (caseous necrosis).  The organism is able to survive in an anoxic environment and therefore continues to live, resulting in further and further inflammation. Down the microscope one sees chronic inflammation with fibrosis. Until the necrotic tissue has eaten into bronchial wall the patient is not infectious.  Once a bronchus has been breached and the material can enter into the lumen, the patient is able to cough up infectious material (open TB) and is now infectious. If the necrotic material breaches blood vessels, infectious material can seed through the cardiovascular system resulting in miliary tuberculosis.

In summary, the highly resistant organism chronically stimulates the immune response, particularly the cell mediated component, resulting in extensive tissue necrosis (innocent bystander damage).  The patient will have formed antibodies against the organism but it a minimal role in the destruction of the bacteria. The process is predominantly involving the cell mediated component of the immune response.

TYPE 5.

This can be considered part of  TYPE 2. In this situation preformed stimulatory antibodies are made against an autoantigen. For example in Graves disease of the thyroid, antibodies are made against the TSH surface receptor of the thyroid follicular cells, causing chronic stimulation and release of T3 and T4, giving rise to thyrotoxicosis. Why these antibodies are formed is unknown.  However the primary problem is probably a breakdown in T cell regulation, allowing a clone of B-cells to form plasma cells that produced a specific antibody. This is part of the humoral immune response system.

Hypersensitivity memory aid

Summary

In this tutorial I have tried to outline the main reasons we have an immune response; to combat external infections and destroy internal mutations of self. Approximately half of our T-cells are monitoring mutations in our own cells.  The immune response is made up of several components involving cells and proteins that can work independently as well as in conjunction with each other. The basic mechanisms are relatively simple but how the whole system is kept in control is very complex and poorly understood. The hypersensitivity examples are used to illustrate how the immune response can work in certain situations but in almost all cases the humoral and cellular component work together. It has a major role in acute and chronic inflammation.  The immune response is also important in understanding transplantation and the newer treatments for malignancy.

A/Prof John Pedersen


Comments are closed.

© Copyright TissuPath. All rights reserved 2017