Specific Immune Response: T Lymphocytes
- 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
- T-lymphocytes (T cells)
Maturation of T-lymphocytes
- 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
Mature T lymphocytes have many different types of surface receptor, each of which is complementary to a different antigen
T lymphocytes in the immune response
- 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
- T cells with T cell receptors that are complementary to the specific pathogenic antigen bind to the APC
- 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 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
- T killer cells patrol the body in search of antigen-presenting body cells
- 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
- T helper cells
Activated T cells divide by mitosis to produce clones. Cloned T helper cells produce chemicals that activate B cells while cloned T killer cells destroy infected body cells.