Human immune system

Introduction

Previous chapters looked at pathogenic microorganisms and their interaction with human bodies. This chapter continues the subject but will discuss human-pathogen interaction on a cellular level. It will also look at the human immune system, immune response and vaccination.

Immunity and immune response

All living organisms have a natural or acquired resistance mechanism called immunity. The immune response is the general reaction of the body to substances that are foreign to it. Immunity can be active or passive. Active immunity is generated by the body in response to stimulation by a disease-causing organism or vaccine. Passive immunity is transferred to the patient passively rather than formed by the patient. An organism's passive immunity can be generated by transferring antibodies from another organism.

The immune response as such consists of two steps: non-specific and specific.

Non-specific defences

A non-specific response is the first step of the immune response. At this stage, an organism senses that something is wrong and 'switches on' all its defence mechanisms. The body'sfirst line of defence against pathogens uses mostly physical and chemical barriers such as sweat, skin, tears, mucus or stomach acid. Non-specific responses block the entry and spread of disease-causing agents. A highly specific immune response is 'custom designed' for a particular threat.

Specific defences

Different body invaders generate different immune responses. For each type of pathogen our immune system produces cells that are specific to that particular pathogen. A specific immune response is more effective than a non-specific response. A specific response has an element of memory. When we become infected by the same pathogen again, our body will know what to do because it has dealt with this problem before.

Antibodies

Human immunity results from the production of antibodies specific to a given antigen.

An antibody is a protein produced by the body's immune system that recognises and helps fight infections and other foreign substances in the body. Antibodies are Y-shaped molecules composed of two identical long protein molecules and two identical short protein molecules. Antibodies are a type of protein molecule known as immunoglobulins. An antibody's' 'job' involves the recognition and deactivation of antigens. See image 1.

Antigens

An antigen is a substance foreign to the body that stimulates production of antibodies by the immune system. Antigens might include foreign objects and proteins, bacteria, viruses, pollen and transplant organs. Antibodies bind to the antigens trying to kill them or render them inactive. See image 2.

Immune system

The immune system includes all parts of the body that help in the recognition and destruction of foreign materials. White blood cells, macrophages, lymphocytes, bone marrow, lymph nodes, tonsils, thymus and spleen are all part of the immune system. There are a few main groups of cells that defend our bodies. They are:

Macrophages - are white blood cells that continually search for foreign substances. When macrophages find them, they engulf and destroy them.

T-cells - are macrophages that become activated when they find antigens. Activated T cells identify and activate B cells, destroy antigens and 'switch off' the immune response of the body when the antigens are destroyed.

B-cells - form plasma cells and B memory cells. Plasma cells produce and release antibodies into the blood. B memory cells live for months or years and are part of the immune memory system.

Our body can distinguish between its own substances and foreign substances. See image 3.

General defences

Our skin and mucous membranes are the antigen barriers of our bodies. The skin is a passive barrier to infectious agents such as bacteria and viruses. Tears and saliva produce enzymes that break down bacterial cell walls. Skin glands also make chemicals that stop the growth of bacteria. Mucus membranes lining the respiratory, digestive, urinary and reproductive tracts produce mucus that forms another barrier.

When microorganisms penetrate the skin or respiratory, digestive or urinary tracts they cause tissue inflammation. Damaged body cells release chemicals called histamines. Histamines increase blood flow into the affected area, causing the area to become heated and reddened. This heat makes an antigen's life very difficult because most pathogenic organisms prefer our normal body temperature, which is 36.6 C. That is why we 'get temperature' when we are sick. Increased body heat promotes healing, raises the mobility of white blood cells and increases the metabolic rate of nearby cells.

Vaccination

Secondary immunity is the resistance to an antigen the second time it appears. This type of immunity results from production of memory B and T cells during the first exposure to the antigen. A second exposure to the same antigen produces a more massive and faster response. This secondary response of our immune system is the basis for vaccination. A vaccine is a mixture of dead or weakened pathogens that is used to trigger the formation of antibodies without causing the actual disease. The injection of the vaccine into the body is called vaccination.

Immunodeficiency diseases

Immunodeficiency diseases result from the lack, or failure, of one or more parts of the immune system.

Severe Combined Immunodeficiency (SCID) results from a complete absence of the cell-mediated and antibody-mediated immune responses.

Acquired Immunodeficiency Syndrome (AIDS) is a collection of disorders resulting from the destruction of T cells by the Human Immunodeficiency Virus (HIV).