White blood cells
White cells (leucocytes) defend the body
from viruses, bacteria and parasites; at
such times, cell numbers will be raised.
High concentrations are also found in
rheumatoid arthritis and cancer, and after
surgery. There are five types of white cells:
» Neutrophils: making up to 70% of the
white cell count, these recognise,
attack and destroy bacteria;
» Lymphocytes: the second most
common white blood cell (approximately
20-25% of the white cell count),
are divided into two types – B lymphocytes
make antibodies, while T lymphocytes
destroy cells infected with viruses;
» Monocytes: these have several functions,
including bacteria removal, and
are active in inflammation and in repair
of damaged tissues;
» Eosinophils and basophils: these cells
have roles in hypersensitivity and
allergy.
White cells defend the body from
most microbial pathogens through two
processes:
» Inflammation: this develops rapidly
and is associated with high neutrophil
numbers, but can lead to the body
attacking its own tissues, leading to
chronic inflammation;
» An immune response, where lymphocytes
are active: this develops slowly,
over days or weeks, and is focused on
the invading pathogen.
Inflammatory and immune responses
often cooperate. For example, lymphocytes
make antibodies that bind to bacteria
and yeast pathogens, making them more
palatable to the neutrophils and monocytes,
which aids their removal. Infections
occur when either or both these processes
become impaired. Antibodies can also
cause autoimmune diseases such as rheumatoid
arthritis and thyroiditis