Download 2.-LYMPHOCYTE-info

B lymphocytes
Lymphocytes are a type of white blood cell. There are a number of different types
of lymphocyte; two of the most important are B lymphocytes and T lymphocytes.
B lymphocytes are also called B cells. They are formed in the bone marrow of
mammals. B lymphocytes play an important role in the humoral immune system.
B cells play three important roles in the immune response:
1.
2.
3.
they produce antibodies in response to specific antigens
they perform the role of antigen-presenting cells
they develop into memory B cells.
Production of antibodies
Each B cell can produce a specific antibody that can recognise a specific antigen
from a pathogen or toxin. The antibody will bind to the antigen, forming an antigen –
antibody complex.
The antigen–antibody complex has two modes of action:
 It may inactivate the pathogen/toxin, rendering it susceptible to phagocytosis.
 It may stimulate a response, resulting in cell lysis.
Antigen-presenting cells
B cells can identify specific antigens and present them to T cells. T cells can then
destroy the pathogen.
Memory B cells
Memory B cells exist long after the infection has subsided. Should the same
antigen reappear in later life the memory B cells will be on hand to stimulate the
appropriate immune response.
T lymphocytes
Lymphocytes are a type of white blood cell. There are a number of
different types of lymphocyte; two of the most important are B
lymphocytes and T lymphocytes.
T lymphocytes are also called T cells. They are formed in the thymus
gland – hence their name. T lymphocytes play an important role in the
immune system and they account for more than four -fifths of circulating
lymphocytes.
There are two main groups of T lymphocytes: cytotoxic T cells and helper
T cells.
Cytotoxic T cells
The cytotoxic T cells are also called killer T cells. They recognise
antigens on infected cells, bind to the antigen and can destroy the cells
by apoptosis. Apoptosis is a form of programmed cell death – the cell is
instructed to destroy itself.
Once cytotoxic T cells have completed their job, the majority of cells
undergo apoptosis. However, a few cells become ‘memory cells’, which will
remain on hand to respond should the antigen reappear at some point in
the future.
Helper T cells
Helper T cells are not able to destroy infected cells. Instead they
produce cytokines – a chemical signal that activate B lymphocytes and
phagocytes, which can then act to destroy the infected cell.