4.1.7 The T Lymphocyte Response

Question 1

What type of immune response do T lymphocytes produce?

Answer 1

Lymphocytes and antibodies provide the third line of defence against pathogens
Unlike the first and second lines of defence, the third line is specific
Specific immune responses are slower but more effective than non-specific immune responses

Lymphocytes are
A type of white blood cell
Smaller than phagocytes
Have a large nucleus that fills most of the cell
Produced in the bone marrow before birth
Travel around the body in the blood

There are two types of lymphocytes (with different modes of action)
T-lymphocytes (T cells)
Lymphocytes that mature in the thymus gland
B-lymphocytes (B cells)
Lymphocytes that mature in the bone marrow

Question 2

Describe the maturation of T lymphocytes.

Answer 2

Immature T-lymphocytes originate in the bone marrow

They move to the thymus gland in the chest, which is where they mature

During the process of maturation T lymphocytes (T cells) gain specific cell surface receptors called T cell receptors (TCRs)
These receptors have a similar structure to antibodies and are each complementary to a different antigen
A small number of T cells have the same TCRs, these genetically identical cells are called clones
T cells within each clone differentiate into different types of T cell: T helper cells and T killer cells

There is a very large number of different T cells with different TCRs
This variation allows the T cells to recognise a wide range of foreign antigens
Foreign antigens can be found on the surface of microorganisms, their cell products and toxins

The matured T cells remain inactive until they encounter their specific antigen

Question 3

Describe the role of T lymphocytes in the immune response.

Answer 3

In order to play their role in the immune response T cells need to be activated and increase in number; this process is described below

Antigen presentation
Macrophages engulf pathogens and present the pathogen antigens on their own cell surface membrane
They become antigen-presenting cells (APCs)

Clonal selection
T cells with T cell receptors that are complementary to the specific pathogenic antigen bind to the APC
They are the clones that have been selected for replication
Binding to the complementary antigens causes the T cell to be activated

Clonal expansion
Activated T cells divide by mitosis to produce clones
There are now many T cells in the blood, all of which have specific roles
T helper cells
These cells release chemical signalling molecules known as interleukins (a type of cytokines)
Interleukins causes phagocyte activity to increase
Interleukins is needed to activate B cells
T killer cells
T killer cells patrol the body in search of antigen-presenting body cells
T killer cells attach to the foreign antigens on the cell surface membranes of infected cells and secrete toxic substances that kill the infected body cells, along with the pathogen inside
Perforins secreted by T killer cells punch a hole in the cell surface membrane of infected cells, allowing toxins to enter
T memory cells
Memory cells remain in the blood, meaning that if the same antigen is encountered again the process of clonal selection will occur much more quickly