Antigens, antibodies and active immunity

Diseases and immunity · Immunity · note 1 of 3

Spec 10.1.6, 10.1.7, 10.1.8, 10.1.9, 10.1.10

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Antigens, antibodies and active immunitySpec 10.1.6, 10.1.7, 10.1.8, 10.1.9, 10.1.10

Each pathogen has its own antigens. Antigens are molecules on the surface of the pathogen, and they have specific shapes.

Antibodies are proteins made by white blood cells (lymphocytes). An antibody has a shape that is complementary to one specific antigen, so it fits that antigen and no others. This means that a different antibody is needed for each pathogen.

Antibodies bind to antigens. This leads to either direct destruction of the pathogens or marking of the pathogens so that phagocytes destroy them.

Active immunity is defence against a pathogen by antibody production in the body. It is gained in two ways:

after an infection by a pathogen (the body makes antibodies and recovers)
by vaccination

Graph of antibody concentration over time showing a slow, small primary response and a fast, larger secondary response — Memory cells made after the first exposure make the secondary response faster and bigger than the primary response.

A pathogen with antigens on its surface: an antibody with a complementary shape binds to an antigen, while an antibody with a different shape does not fit. Two outcomes are shown: the pathogen is destroyed directly, or antibodies mark the pathogen and a phagocyte engulfs it. — Each antibody is complementary to one antigen. Binding leads to direct destruction of the pathogen or marks it for destruction by phagocytes.

Exam tip:

Use the word 'complementary' for the shape of antibody and antigen. An antibody is specific to one antigen.