Lymphocytes
- They come from the bone marrow
- Found only in vertebrates immune systems
- Part of the adaptive immune system
- All lymphocytes originate, during haematopoiesis from a common
lymphoid progenitor before differentiating into their distinct lymphocyte
types.
- Migrate to parts of the lymphatic system such as the lymph nodes, spleen,
thymus, tonsils, peyers patches (in intestine)
- two main types of lymphatic cells, T cells and B cells also Natural killer
cells (more important in the innate immune system)
- lymphatic system also involves a transportation system - lymph vessels -
for transportation and storage of lymphocyte cells within the body
- The lymphatic system feeds cells into the body and filters out dead cells
and invading organisms such as bacteria.
- The lymphocyte cells travel through your body until they find an antigen
of the right size and shape to match their specific receptors
How do lymphocytes differ from Phagocytic cells?
1. Lymphocytes re-circulate through the body looking for antigens while
phagocytic cells are stimulated to migrate towards the antigens along a
chemotactic gradient
2. Lymphocytes are antigen specific due to their specific surface receptors;
phagocytes eat anything foreign that they find
3. Recognition of specific antigens causes proliferation (To grow or multiply by
rapidly) or cell activity e.g. The secretion of cytokines
4. Lymphocytes maintain a memory of the antigen allowing faster second degree
response; phagocytes do not have any recognition thus they have no memory.
Therefore, their secondary response would be no quicker if they worked alone.
Natural killer T cells (NKT cells)
A special kind of lymphocyte that bridges the adaptive immune system with the innate
immune system.
NKT cells recognize glycolipid antigen presented by a molecule called CD1d.
Once activated, these cells can perform functions ascribed to both Th and Tc cells (i.e.,
cytokine production and release of cytolytic/cell killing molecules).
The B cells
B cells migrate to the spleen and mature into B lymphocytes
When a B cell is in resting stage it has IgM and IgD on the cell wall
The B lymphocyte cell searches for antigen matching its receptors. If it finds such antigen it
connects to it, and inside the B cell a triggering signal is set off
When the B cell is stimulated an Ig class switch takes place and replaces IgM and IgD with
IgG as the new surface receptor
B cell now needs proteins produced by helper T cells to become fully activated.
When this happens, the B cell starts to divide to produce clones of itself. (This is the reason
that lymph nodes swell during an infection – B cells are splitting faster than they are leaving)
During this process, two new cell types are created, plasma cells and B memory cells.
o The plasma cell is specialized in producing a specific protein, called an antibody that will
respond to the same antigen that matched the B cell receptor.
o Antibodies are released from the plasma cell so that they can seek out intruders and help
destroy them. Plasma cells produce antibodies at an amazing rate and can release 100
000/min for up to 5 days.
o When the Y-shaped antibody finds a matching antigen, it attaches to it.
o Opsonisation occurs
The attached antibodies serve as an appetizing coating for eater cells such as the
macrophage.
Antibodies also neutralize toxins and incapacitate viruses, preventing them from infecting
new cells.
Bacteria and other pathogens covered with antibodies are also more likely to be attacked
by the proteins from the complement system.
o Memory Cells are the second cell type produced by the division of B cells. These cells have
a prolonged life span and can thereby "remember" specific intruders.
o They have IgG on their surface
Not having to go through the class switch helps to make the secondary
response faster
T cells
o T cells are white blood cells known as lymphocytes
o All T cells originate from hematopoietic stem cell in the bone marrow.
o Play a central role in cell mediated immunity (adaptive immune system)
o T cells can also produce memory cells with an even longer life span than B memory cells.
o T cells migrate to and mature in a distinct organ, called the thymus.
o Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs
(e.g. the spleen and lymph nodes) where they survey for invading pathogens and/or tumour
cells.
o They can be distinguished from other lymphocyte types, such as B cells and natural killer
cells by the presence of a special receptor on their cell surface called the T cell receptor
(TCR).
Cytotoxic T cells (TC cells, CD8 T cells, or CTLs)
NOTE: All nucleated cells have MHCI
Destroy virally infected cells and tumour cells, and are also implicated in transplant
rejection.
These cells are also known as CD8 T cells, since they express the CD8 glycoprotein at
their surface.
associated with MHC class 1 They can look into a cell and decide to kill it or not
CD4 T Cells With MHC Class II
NOTE: Only B cells, macrophages and dendritic cells have MHC II
o In resting stage they are called TH0
Helper T cells (TH cells)
o Once activated, they divide rapidly and secrete small proteins called cytokines
that regulate or "help" the immune response.
o Depending on the cytokine signals received, these cells differentiate into TH1 or
TH2
TH1 produces INF gamma to activate macrophages to mature
TH2 produces IL4 to stimulate B cells to divide and produce antibodies
o Associated with MHC class II.
Note: if one form is activated the other form is turned off
Memory T cells
o A subset of antigen-specific T cells that persist long-term after an infection has
resolved.
o They quickly expand to large numbers of effector T cells upon re-exposure to
their cognate antigen, thus providing the immune system with "memory" against
past infections.
o Memory T cells comprise two subtypes: central memory T cells (TCM cells) and
effector memory T cells (TEM cells).
o Memory cells may be either CD4 or CD8.
Regulatory T cells (Treg cells)
o formerly known as suppressor T cells
o Crucial for the maintenance of immunological tolerance.
o Their major role is to shut down T cell-mediated immunity toward the end of an
immune reaction and to suppress auto-reactive T cells that escaped the process of
negative selection in the thymus.
o Main cytokine is IL10
o Two major classes of CD4 regulatory T cells have been described, including the
naturally occurring Treg cells and the adaptive Treg cells.
The binding of CD 8 to MHC 1 and CD4 to MHC II ensures that Tc and TH cells interact
the way they should and perform the proper functions.