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Biology Unit 4
Topic 6: Infection, Immunity and Forensics
13. Explain the roles of antigens and antibodies in
the body's immune response including the
involvement of plasma cells, macrophages and
The specific immune response relies on the lymphocytes, of which there are two main kinds, each
with a number of sub-types. Both types respond to foreign (non-self) antigens, such as proteins on
the surface of bacteria and viruses. Macrophages are also involved, engulfing bacteria and
displaying the non-self antigens. They alert the immune system to the presence of the foreign
antigens. When any cell in the immune system displays antigens in this way, it is called an
An antigen is a substance that stimulates the production of an antibody when it gets into the body.
Antigens are often chemicals on the surface of a cell such as proteins, glycoproteins or
carbohydrates. They can also be toxins made by bacteria, or sometime are whole microorganisms
such as bacteria or viruses.
Macrophages function in both non-specific defence as well as help initiate specific defence
mechanisms. Their role is to engulf and digest cellular debris and pathogens, either as stationary or
mobile cells. They also stimulate lymphocytes and other immune cells to respond to pathogens. They
are specialised phagocytic cells that attack foreign substances, infectious microbes and cancer cells
through destruction and ingestion.
An antigen-presenting cell or accessory cell is a cell that displays foreign antigen complexes with
major histocompatibility complex (MHC) on their surfaces. T-cells may recognise these complexes
using their T-cell receptors. These cells process antigens and present them to T-cells.
When the body is exposed to a pathogen, like a virus, the immune system creates antibodies against
the pathogen. Those antibodies stay in the blood. The next time the body is exposed to that
pathogen, the antibodies attack it before it can make us sick. When pathogens arrive, the immune
system activates antibodies to deal with the invader.
Text Book: p. 96, 100-101