Infodump 4
Infodump 4
The Common Lymphoid Progenitor (CLP) gives rise to the 'Lymphocytes'. The lymphocytes themselves are divided into 3 types: B lymphocytes (B cells), T lymphocytes (T cells) and Natural Killer (NK cells). NK cells, as mentioned in the previous infodump (link it), are part of the innate immune system while the B and T cells form the adaptive immune system.
B and T cells are indistinguishable under a microscope due to their morphological similarities. Thus to differentiate these cell types, another point of difference must be exploited. The surface proteins expressed on the immune cells are unique to a particular cell types. These surface proteins, referred to as Cluster of Differentiation (CD) form the basis of identifying and distinguishing between the various lymphocyte populations.
Additionally, B and T cells express an antigen-specific receptor called as B cells receptor (BCR) and T cell receptor (TCR). These receptors exhibit an expansive diversity in their ability to identify any foreign antigen that enters the body. However, an individual lymphocyte contains identical copies of a particular receptor. When a lymphocyte is 'selected' for eliminating an antigen, it proliferates and differentiates. All the daughter cells arising from this proliferation have identical receptors as the parent cell and thus the same antigen specificity. The populations that arise from a founding cell are thus referred to as 'Clones'.
One of the notable properties of the adaptive immune system is 'Memory'. Upon activation of B and T cells, they proliferate and differentiate into two cell types: Effector cells and Memory cells. The effector cells are responsible for carrying out functions that ultimately lead to the elimination of the pathogen. The memory cells persist in the host and upon a rechallenge mount an accelerated and stronger response against the pathogen. The first encounter with an antigen is known as Primary Response and the re-encounter is known as Secondary Response. B and T cells before their first encounter are known as Naïve cells.
B lymphocytes differentiate from the CLP and mature in the bone marrow in mammals.
These lymphocytes display BCR which is a membrane-bound antibody molecule which binds to antigens.
Activated B cells differentiate into Plasma cells which lose their surface antibody expression and become specialized for antibody secretion.
One plasma cell has the capacity to secrete a few hundred to thousand antibodies per second.
Plasma cells do not divide.
Antigen-binding ability can be improved by a process known as Somatic Hypermutation (Increased levels of mutations in the antibodies. Will be discussed in future infodumps).
Antibodies or Immunoglobulins are characterized into different classes based on their structure. The classes each have their own distinct function. B cells can produce different antibodies in accordance to the requirement by a process known as Class Switching (will be discussed in future infodumps).
T lymphocytes arise from CLP and migrate to the Thymus for maturation.
T cells express TCR but unlike BCR, which identify soluble or particulate antigen, these only identify peptides of antigens processed and presented by cell membrane proteins called Major Histocompatibility Complex (MHC) molecules.
MHC are glycoproteins found on cell membranes that bind to antigens. These presented antigens are available for identification by browsing T cells. Two classes of MHC molecules:
MHC Class I - expressed by almost all nucleated cells of vertebrate species.
MHC Class II - expressed by Professional Antigen-Presenting Cells (APCs)
T lymphocytes can be divided into two categories:
T helper (TH) cells - Presence of CD4 membrane glycoproteins.
Recognize antigens in complex with MHC class II
Upon activation, proliferate and differentiate into one of the following TH cell subsets.
TH1 cells - regulate response to intracellular pathogens
TH2 cells - regulate response to extracellular pathogens
TH17 cells - secrete IL-17 (Interleukin-17), may help against fungi
TFH cells (T follicular helper cells) - regulates B cell development in germinal centres and important role in humoral immunity
TREG cells (Regulatory T cells) - inhibit immune response
T cytotoxic (TC) cells - Presence of CD8 membrane glycoproteins
Recognize antigens in complex with MHC class I
Recognize and eliminate cells that display any foreign antigen such as virus-infected cells, tumour cells, and cells of a foreign tissue graft.
Require help from mature TH cells for optimal proliferation and differentiation.
NK cells are lymphoid cells that do not express antigen-specific receptors.
These cells are a component of the innate immune system.
They constitute 5-10% of the lymphocytes in blood.
They can be identified by the presence of the NK1.1 surface marker and cytotoxic granules.
They eliminate various abnormal cells such as the virus-infected cells and some tumour cells.
The identification of these abnormal cells is not specific. NK cells detect the absence of MHC class I receptor, which as previously established are expressed on almost all vertebrate cells, but are downregulated in case of some tumour and virus-infected cells.
The absence of the receptor triggers the release of the cytolytic granules result in killing of the target cell.
NK cells also express receptors that can attach to antibodies. These antibodies can then bind to pathogen or antigens presented on infected cells resulting in the release of cytolytic granules from the NK cell to eliminate the infected cell.
NKT cells share features with both NK and T cells.
Like T cells, they have TCRs but it recognizes specific lipids and glycolipids presented by CD1 (molecule related to MHC proteins).
Like NK cells, they have antibody receptors and release cytolytic granules to kill target cells.
However, they also release cytokines that can enhance and suppress the immune system.
The exact role and function has not been clearly understood.
Their involvement with human asthma, inhibition of cancer and autoimmunity have been speculated.
The next infodump will deal with the organs of the immune system.