Specific immunity

Antibody attaches to antigen and destroys bacteria

Fig 6.8 The antibody defence system

Immunity involves interaction between two types of molecule — an antigen and an antibody.

An antigen is any substance that the body regards as foreign or potentially dangerous and against which it produces an antibody. An antibody is a specific protein produced to destroy or suppress antigens.

Antibodies circulate in the blood and tissue fluid, killing germs or making them harmless. Antibodies also neutralise poisonous chemicals called toxins which germs produce. Specific immunity involves very specific responses to each identified foreign substance and calls on special memory cells to help if the invader reappears. This is the ability to recognise certain antigens and destroy them. The body must be able to identify which substances are capable of causing a threat before any type of response can be initiated.

How antibodies work

Antibodies work in many different ways. Some neutralise the antigens when they combine with them and prevent them from carrying out their effects. Others may lyse (destroy) the cell on which the antigen is present. When antibodies are bound to antigens on the surface of bacteria, they attract other white blood cells like macrophages to engulf them.

The key cells of specific immunity are a specialised group of white blood cells called lymphocytes. They are capable not only of recognising foreign agents but remembering the agents they have encountered and, therefore, are able to react more rapidly and with greater force if they encounter the agent again.

The immune response

There are two types of immune response produced by different types of lymphocytes:

• Humoral immunity — this involves the B-lymphocytes which produce free antibodies that circulate in the bloodstream.

• Cell-mediated immunity — this is effected by the helper T-cells, suppressor T-cells and natural killer (NK) cells that recognise and respond to certain antigens to protect the body against their effects.

Lymphocytes develop in the following three ways:

• T-cells begin in the bone marrow and grow in the thymus gland. They are able to recognise antigens and respond by releasing inflammatory and toxic materials. Specialised T-cells also regulate the immune response, either by amplifying the response (T4 cells) or by suppressing the body's response (T8 cells). Some T-cells develop into memory cells and handle secondary response on re-exposure to antigens that have already produced a primary response.

• B-cells grow and develop in the bone marrow. B-cells contain immunoglobulin, an antibody that responds to specific antigens. Some B-cells modify and become non-antigen specific which means that they have a greater ability to respond to bacterial and viral pathogens. Some B-cells become memory cells and are able to deal with re-exposure to antigens.

• A type of lymphocyte that does not develop the same structural or functional characteristics as the T-cells or B-cells are the natural killer cells (NK) cells. They also develop in the bone marrow and when mature can attack and kill tumour cells and virus-infected cells during their initial developmental stage before the immune system is activated.

Primary and secondary responses

The initial response of the body on first exposure to antigens is known as the primary response. It normally takes about two weeks after exposure to the antigen for antibody levels to peak. This is due to the fact that B-cells have to become converted to plasma cells that secrete antibodies specifically against the antigen.

If the individual is exposed to the antigen the second time, the presence of memory cells stimulates rapid production of antibodies and this is known as the secondary response. The antibody levels are much higher than the primary response and remain elevated for a very long time. Secondary response can occur even if many years have elapsed since the first exposure to the antigen.

Immunisation

The body may be artificially stimulated into producing antibodies and this is known as immunisation. This prepares the body to ward off infection in advance and is carried out by inoculating an individual with a vaccine (a liquid containing antigens powerful enough to stimulate antibody formation without causing harm). Vaccines have been developed against many diseases including diphtheria, polio, tetanus, whooping cough and measles.

Allergy

Under certain circumstances abnormal responses or allergic reactions may occur when a foreign substance, or antigen, enters the body. An allergic reaction can only occur if the person has already been exposed to the antigen at least once before and has developed some antibody to it.

The type and severity of an allergic reaction depends upon the strength and persistence of the antibody screen evoked by previous exposure to the antigen. These antibodies are located on the cells in the skin or mucous membranes of the respiratory and gastro-intestinal tracts. Typical antigens include pollen, dust, feathers, wool, fur, certain foods and drugs.

The reactions may cause symptoms of hay fever, asthma, eczema, urticaria and contact dermatitis. If there is much cellular damage, excessive amounts of histamine may be released causing circulatory failure (anaphylaxis). Anaphylactic shock is an extreme and generalised form of allergic reaction whose widespread release of histamine causes swelling (oedema), constriction of the bronchioles, heart failure, circulatory collapse and may even result in death.

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