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Immunity (derived from immunitas: Latin for exemption from civic duties and prosecution) means protection from disease and especially infectious disease. Cells and molecules involved in such protection constitute the immune system and the response to introduction of a foreign agent is known as the immune response. Not all immune responses protect from disease; some foreign agents, such as the allergens found in house dust mite, cat dander or rye grass pollen, cause disease as a consequence of inducing an immune response. Likewise some individuals mount immune responses to their own tissues as if they were foreign agents. Thus, the immune response can cause the autoimmune diseases common to man such as multiple sclerosis, diabetes, rheumatoid arthritis or myasthenia gravis. Most individuals do not suffer from autoimmune disease because they have developed tolerance towards their own (self) tissues.
Innate and Adaptive Immunity
The normal individual has two levels of defence against foreign agents. The first type is present in neonatal animals and in invertebrates namely natural or innate immunity. This type of immunity is sometimes referred to as non-specific but broadly specific would be a better description. The second type of immunity is adaptive or acquired immunity and is confined to vertebrates.
Innate (or natural) immunity
This is made up of several components.
The acute inflammatory response which has been described in previous lectures is a key part of the innate immune system. Many infections, especially where small wounds are the route of entry, are eliminated by the combination of complement and recruitment of phagocytes, which flow from the acute inflammatory response.
A defining aspect of the innate immune system is that it carries no memory of an encounter with a foreign organism.
What is an antigen?
An antigen is defined as "anything that can be bound by an antibody". This can be an enormous range of substances from simple chemicals, sugars, small peptides to complex protein complexes such as viruses. The small antigens are not, however immunogenic in themselves and need to be coupled to a carrier to elicit an immune response. Such small antigens are referred to as haptens.
In fact antibodies interact specifically with relatively small parts of molecules. These are known as antigenic determinants or epitopes. Sometimes the epitope is composed of a string of amino acids as might be found in a short peptide, such epitopes are said to be linear. Other epitopes are formed by more complex 3-dimensional structures present only as part of a native protein, such epitopes are called conformational.
The second level of defence increases in
strength and effectiveness with each encounter. The foreign agent is
recognised in a specific manner and the immune system acquires
memory towards it.
The first encounter with an antigen is known as the primary response. Re-encounter with the same antigen causes a secondary response that is more rapid and powerful.
Antibodies work in three ways.