The adaptive immune system Flashcards
The two main cell types involved in the adaptive immune response are what?
T lymphocytes and B lymphocytes
While natural killer cells do arise from the lymphoid lineage, they are more involved in the innate immune system
what can T and B lymphocytes be further divided into?
T lymphocytes can be further subdivided into CD4+ T helper lymphocytes, and CD8+ cytotoxic T lymphocytes. Some B lymphocytes mature into plasma cells and are responsible for antibody production.
where do T and B cells complete their maturation?
T lymphocytes and natural killer cells. B lymphocytes mature within the bone marrow, whereas T lymphocytes move from the bone marrow and complete their maturation within the thymus gland.
During T cell development within the thymus, the _ ____ ________ (TCR) is formed, a structure unique to T lymphocytes
The majority of T cells express a TCR composed of alpha and beta chains. Within these protein chains are ‘complementarity determining regions’ (CDRs), which is the name given to the part of the TCR that binds to antigens
T cell receptor
what are the function of CD4+ and CD8+ lymphocytes?
Within the introductory module, we introduced the idea that CD4+ T lymphocytes recognise antigens in association with MHC II molecules and have a T helper function, whilst CD8+ T lymphocytes interact with antigens in association with MHC I molecules and have a cytotoxic function
the thymocytes undergo positive and negative selection before being released from the thymus as naïve CD4+ or CD8+ T lymphocytes
what is positive and negative selection?
Positive selection involves selecting T lymphocytes with T cell receptors capable of interacting appropriately with host MHC molecules. If the double positive thymocytes are able to interact with the MHC, they survive. If they are unable to interact with the MHC, they are destroyed by apoptosis
Negative selection involves identifying thymocytes that interact too strongly with self antigens within the thymus. Thymocytes that react too strongly, are destroyed by apoptosis
what happens to T cells after positive and negative selection?
Thymocytes that survive both positive and negative selection differentiate into single-positive T cells (either CD4+ or CD8+)
This depends on whether their TCR recognizes an MHC class I-presented antigen or an MHC class II-presented antigen
These single positive T lymphocytes are then released from the thymus into peripheral tissues.
CD8+ T cells, once primed by antigen-presenting cells, are otherwise known as cytotoxic T lymphocytes, or Tc cells
what is their function?
The function of these cytotoxic T cells includes recognizing and destroying virally infected host cells, host cells that have become malignant, or any other host-cells showing signs of damage via their MHC I expression. Once activated, cytotoxic T lymphocytes can kill a cell using 3 different mechanisms
Another subset of T lymphocyte are the CD4+ helper T lymphocytes (Th)
what is their function?
Helper T lymphocytes help activate other immune cells by releasing T cell cytokines to either stimulate or regulate the immune response
T helper cells are also responsible for B lymphocyte antibody class-switching, the activation and growth of cytotoxic T lymphocytes and they maximize the activity of phagocytes
There are various different types of helper T lymphocyte, including: Th1, Th2, and Th17. Each T helper cell has a slightly different function
what is their function?
The final type of T lymphocyte that will be discussed is the regulatory T cell
what are they?
During selection of T cells in the thymus, some T cells that recognise self-antigens are not deleted and differentiate into regulatory T cells (or Tregs) that are specific for that antigen
Tregs are responsible for suppressing the activation, proliferation and cytokine production of CD4 and CD8 lymphocytes
In addition, T regs control the response to self-antigens, and in this way monitor the process of self-tolerance
T regs secrete IL-10 and Transforming growth factor-beta (TGF-b), which are anti-inflammatory and help to suppress the immune response.
what is the function of B cells?
B lymphocytes are responsible for humoral immunity as part of the adaptive immune response
Humoral immunity acts primarily through the actions of antibodies (or immunoglobulins) directed against specific antigens
Antibody-antigen interactions are a bit like a lock and key mechanism – with their binding sites complementing each other exactly
Antibodies can be found in two forms; what are they?
a soluble form secreted by plasma cells that are found free-floating in the bloodstream, or in a fixed, membrane bound form attached to the surface of a B cell. This membrane bound antibody can also be referred to as the B cell receptor
whata re the functions of the different types of antibodies?
- IgM are the largest antibodies and form an important part of the primary antibody response to an antigen. As a result of their size and increased number of binding sites, they are much more efficient at activating complement
- IgA antibodies are dimeric and present within secretions such as tears and breast milk
- IgG antibodies form the main component of the secondary antibody immune response to an antigen, and are the only isotype of antibody able to cross the placenta
- IgE antibodies form part of the immune response to parasites, however they are also involved in the pathogenesis of allergic reactions
here are multiple ways that antibodies are involved in antigen neutralization and immune system activation
What are they?
1) They can bind to antigens directly to form immune complexes, which can be cleared through filtration of the blood in organs like the liver and spleen
2) Antibodies also act as opsonins for phagocytosis
3) Antibodies are capable of activating the complement cascade via the Classical pathway
4) Antibodies can bind to target cells and iniate a non-phagocytic cell-mediated destruction of these cells. This is called Antibody-dependent cellular cytotoxicity (ADCC)
