What is an antigen?
1. 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.
2. 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.
3. Requirement of Antigen to cause immune response are: Non-self, Complex in structure, should more then 5 KD in size and must have atleast on epitope.
4. In fact antibodies interact specifically with relatively small parts of molecules. These are known as antigenic determinants or epitopes.
5. 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.
Adaptive immunity
1. The second level of defence increases in strength and effectiveness with eachencounter. The foreign agent is recognised in a specific manner and the immune
system acquires memory towards it.
2. 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.
3. Acquired immunity is a useful evolutionary adaptation because it improves the efficacy of the innate immune response by focusing the response to the site of invasion/infection as well as providing additional effector mechanisms that are unique to lymphocytes.
4. The difference between innate and acquired immunity lies in the antigen specificity of lymphocytes. This property is conferred upon lymphocytes by the expression of cell surface receptors that recognise discrete parts of the antigen known as antigenic epitopes.
5. The cell surface receptor of B lymphocytes, (derived and mature in Bone marrow in mammals or the Bursa of fabricius in chickens) is an immunoglobulin molecule which, when secreted by the B cell, is known as an antibody.
6. Immunity provided by Immunoglobulins (antibodies) is termed as humoral immunity(Humor=fluid/blood)
Antibodies work in three ways
1. Agglutination: Antibodies bind to antigens to produce large insoluble complexes, which render them harmless and facilitate their destruction by other cells of the immune system.2. Opsoniation: IgG molecules coat the surface of antigens and this stimulates their
recognition and digestion by phagocytes.
3. Complement mediated cell lysis: Complement system is a group of enzymes.
They are triggered by IgM - IgG bound to the surface of foreign cells. The activity of
some of these enzymes leads to the formation of pores on the plasma membrane ofbthe invading cell causing them to lyse.
Structure of Antibody: Antibodies have two ends. One end interacts with the antigen (the variable part) leaving the other (constant ) end free to interact with the
immunoglobulin receptors on these cells. During an immune response, a complex lattice of interlinked antigens and antibodies, known as an immune complex, will present an array of constant regions which can activate the various cells mentioned above through ligation of their immunoglobulin receptors.
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