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The Immune System

Definitions:

• Immunity - can be defined as the capacity to recognise the intrusion of material foreign to the body and to moblise cells and cell products to remove that sort of foreign material with great speed and effectiveness.
• Endemic - is a disease which is always present at low levels.
• Epidemic - when the number of cases of a disease increase significantly.
• Vectors - orgainisms which carry pathogens between other organisms.
• Infectious - a disease which can be passed between hosts.

Defence Mechanisms

• Natural Barriers - Skins, Mucus Linings and Blood Clotting.
• The human body is an idea incubator for micro organisms. Many live in or on our bodies (comensals) causing no harm and benefiting. Pathogens are disease causing micro organisms and enter in two ways, either through the skin or natural openings.
• The skin is an effective barrier due to its think continuous carotonised layer.
• Micro organisms can be washed off easily and skin can flake off which helps to prevent a build up of bacteria.
• Lysozyme in the eyes breaks down bacteria cell walls.
• Invasion only occurs when skin is broken.

Antigens

Antigens are molecules that can form antibodies. All cells possess antigens in their cell surface membrane which act as markers enabling cells to recognise each other.

Antigens are usually proteins or glycoproteins, although any complex can be antigenic. The body can then distinguish between local and foreign cells.

Antibodies

An antibody is a substance produced by lymphocytes in the presence of a specific antigen. Antibodies then join with the specific antigen to neutralise, inhibit or destroy it.

Types of Immunity

There are two types of immunity, passive and active.

Passive

• Short term and temporary, no memory cells.
• Through artifical means, injection, umbilical cord, etc.

Active

• Long term solution due to memory cells.
• e.g. body producing antibodies against disease.

Immune System Responses

There are two systems of immunity in mammals, cell-mediated immune response and humoral immune response. Both use lymphocytes produced from stem cells in the bone marrow.

Humoral immune response

• Uses B-lymphocytes produced in the bone marrow, where they also mature.
• There are many different types of B-lymphocytes.
• When an foreign antigen enters the blood it combines with a few B-lymphocytes which then divide rapidly through mitosis forming a clone of plasma cells. These then produce mainly antibodies but also memory cells. The memory cells can live for large periods of time, sometime even for life.

Cell-mediated immune response

• Uses T-lymphocytes which are produced in the bone marrow and mature in the thymus gland.
• Once matured T-lymphocytes circulate the body in the blood until it meets an antigen it has the receptor site for. It is then stimulated to divide by mitosis many times forming clones.
• Three types of T-lymphocyte:
  • Killer cells - cause lysis of target cells, will destroy virus infected or cancer cells.
  • Helper cells - activate B-lymphocytes to produce antibodies.
  • Suppressor cells - turn off immune response, e.g. turning off antibody production.

Vaccines

Vaccines work by injecting small amounts antigens from a disease into the body. This triggers an immune response and antibodies are produced. This is the primary response to an intrusion.

As well as antibodies, memory cells are also produced to recognise similar antigens again. If there was a second intrusion with these antigens it would be vastly quicker due to the memory cells and the mammal might not even show any symptoms.

Below is a diagram showing the differences in response time between primary and secondary response.

Monoclonal Antibodies

These are monospecific antibodies that are identical because they are produced by one type of immune cell. They are typically made from fusing myeloma cells with spleen cells from a mouse.

Pregnancy Testing

Monoclonal antibodies detect hCG (human chorionic gonaotropin) in urine around pregnancy, if hCG is present the pregnancy test will turn blue identifying that the subject is pregnant.

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