An immune response is the body's response to a foreign organism (pathogen) entering it. Most pathogens are prevented from entering the body by the skin, or defences such as the mucus that lines the airways. Once a pathogen has entered the body the main forms of defence are the white blood cells (consisting of phagocytes and lymphocytes). The pathogen can be detected in several ways: it can be absorbed by a macrophage, which will then display the surface proteins of the absobed organism on its cell membrane, allowing other white blood cells to recognise it and start an immune response. Cells attacked by the pathogen can release histamine, which attracts neutrophils (a kind of phagocyte) to the area, starting the immune response. B or T lymphocytes can bind on to a foreign suface protein (antigen) and start an immune response).
Each type of
white blood cell performs a specific function:
Macrophages are found in the major
organs and play an important role in initiating the immune response, as mentioned above. Macrophages absorb
pathogens by
phagocytosis, literally engulfing the
organism and digesting them.
Neutrophils patrol the
bloodstream, and are produced in large amounts during an
infection, they are attracted to
cells releasing
histamine and are one of the main parts of the immune response. They perform
phagocytosis on
pathogens.
B-lymphocytes produce
antibodies which perform a number of important functions such as "marking"
pathogens, neutralising
toxins or destroying some
pathogens, they also produce
memory cells, which stay in the bloodstream and are the basis of
immunity.
T-lymphocytes are divided into
t-helper cells which secrete
cytokines which speed up the activities of the
immune system, such as the secretion of
antibodies, and
killer t-cells which secrete toxic substances such as
hydrogen peroxide into affected cells killing the cell and the
pathogens inside.