Immuno cells and Organs - Lymphocytes

Lymphocytes

  • Lymphocytes are white blood cells that are also one of the body's main types of immune cells. They are made in the bone marrow and found in the blood and lymph tissue.
  • It constitute 10-20% immuno cells of body and 99% of the lymph.

Types of lymphocytes

 There are present three type of lymphocyte cell this are;
  1. B - Cell
  2. T - Cell 
  3. Natural killer Cell.

1. B-Cell 

  • The B lymphocyte derived its letter designation from its site of maturation, in the bursa of Fabricius in birds; the name turned out to be apt, for bone marrow is its major site of maturation in a number of mammalian species, including humans and mice
  • Mature B cells are defi nitively distinguished from other lymphocytes and all other cells by their synthesis and display of the B-cell receptor (BCR), a membrane-bound immunoglobulin (antibody) molecule that binds to antigen. 
  • The number of B-Cell receptor on surface is approximately 1.5–3 105.
  • B lymphocytes also can improve their ability to bind antigen through a process known as somatic hypermutation and can generate antibodies of several diff erent functional classes through a process known as class switching. 
  • B cells are primarily responsible for humoral immunity.
  • The other molecules expressed on the membrane of mature B cells are the following:

  1. B220 (a form of CD45) is frequently used as a marker for B cells and their precursors. However, unlike antibody, it is not expressed uniquely by B-lineage cells.

  2. Class II MHC molecules permit the B cell to function as an antigen-presenting cell (APC). 

  3. CR1 (CD35) and CR2 (CD21) are receptors for certain complement products. 

  4. Fc RII (CD32) is a receptor for IgG, a type of antibody.

  5. B7-1 (CD80) and B7-2 (CD86) are molecules that interact with CD28 and CTLA-4, important regulatory molecules on the surface of different types of T cells,including TH cells. 

  6. CD40 is a molecule that interacts with CD40 ligand on the surface of helper T cells. In most cases this interaction is critical for the survival of antigenstimulated B cells and for their development into antibody-secreting plasma cells or memory B cells.

Type of B Cell   

There are present several type of B cell this are

Plasma Blast

  • Plasma blast is a short live,proliferating,Antibody secreting cell which is produce by the differentiation of  B cell in bone marrow
  • This cells are produce in the early of an infection.
  • This are the result of T-cell dependent B-cell activation.

 Plasma Cell 

  • This Cells are  long lived,non proliferating, antibody screening cells which produce by the differentiation of B-cell in bone marrow.
  • Plasma cells are produces from the plasma Blast Cell.
  • This are produce after the infection
  • This are the result of both T-cell dependent or independent activation of B-Cell. 

Lymphoplasmacytoid cell 

  • This are the mixture result of B lymphocyte and plasma cell morphology,and they are thought to be similar to plasma cell.
  • They are found in premalignant and malignant plasma cell.

 Memory B Cell

  • The dormant B cells are arising from the B cell differentiation and circulating into the blood,and they active the immuno system when they detect any antigen.

Regulatory B Cell

  • An immune suppression B cell type they stop the expression of pathogenic pro-inflammatory lymphocyte through the secretion of   IL1 and IL35.

    2. T-Cell            

  • T lymphocytes derive their name from their site of maturation in the thymus.
  • Like B lymphocytes, these cells have membrane receptors for antigen.
  • Although the antigen binding T-cell receptor is structurally distinct from immunoglobulin, it does share some common structural features with the immunoglobulin molecule, most notably in the structure of its antigen-binding site.
  • Unlike the membrane- bound antibody on B cells, though, the T-cell receptor (TCR) does not recognize free antigen.
  • Most T cells recognize antigen only when it is bound to a self-molecule encoded by genes within the major histocompatibility complex (MHC).
  • All T-cell subpopulations express the T-cell receptor, a complex of polypeptides that includes CD3; and most can be distinguished by the presence of one or the other of two membrane molecules, CD4 and CD8. In addition, most mature T cells express the following membrane molecules:
    CD28, a receptor for the co-stimulatory B7 family of molecules present on B cells and other antigen presenting cells.
    CD45, a signal-transduction molecule.

Type of T Cell 


CD4+ Helper Cells
  • This T cells help in the maturation of B cells into plasma cells and memory B cells.
  • They also help activate cytotoxic T cells and macrophages. 
  • They become activated when they are presented with peptide antigens by MHC Class II molecules, which are expressed on the surface of antigen presenting cells (APCs).Once activated, they divide rapidly and secrete small proteins called cytokines that regulate or assist in the active immune response.
 CD8+ Cytotoxic Cells
  •  CD8+ cytotoxic cells cause lysis of virus-infected and tumour cells.
  • They are also involved in transplant rejection.
  • These cells recognize their targets by binding to antigen associated with MHC Class I molecules which are present on the surface of all nucleated cells.

Memory T Cells 

  • Once they come into contact with an antigen naive T cells differentiate into effector cells (CD4+ and CD8+ cells) and memory T cells. Memory T cells are long-lived and can quickly expand to large numbers of effector T cells upon re-exposure to the antigen.
  • They provide the immune system with “memory” against previously encountered pathogens. Memory T cells may be either CD4+ or CD8+. 

Regulatory T cell

This are help in shut down T-cell mediated immunity towards the end of an immuno reaction and to suppress the auto reactive T cell.

Suppressor T Cell

Some t Cells are help to suppress teh humoral and cell mediated immunity this are called suppressor T Cell. 

3. Natural Killer Cells

  • Natural Killer (NK) Cells are lymphocytes in the same family as T and B cells, coming from a common progenitor
  • NK cells are classified as group I Innate Lymphocytes (ILCs) and respond quickly to a wide variety of pathological challenges.
  • NK cells are best known for killing virally infected cells, and detecting and controlling early signs of cancer. As well as protecting against disease, specialized NK cells are also found in the placenta and may play an important role in pregnancy.
  • NK cells secrete cytokines such as IFNγ and TNFα, which act on other immune cells like Macrophage and Dendritic cells to enhance the immune response.

Mechanism

While on patrol NK cells constantly contact other cells. Whether or not the NK cell kills these cells depends on a balance of signals from activating receptors and inhibitory receptors on the NK cell surface.Activating receptors recognise molecules that are expressed on the surface of cancer cells and infected.
cells, and ‘switch on’ the NK cell. Inhibitory receptors act as a check on NK cell killing. Most normal healthy cells express MHC I receptors which mark these cells as ‘self’. Inhibitory receptors on the surface of the NK cell recognise cognate MHC I, and this ‘switches off’ the NK cell, preventing it from killing. Cancer cells and infected cells often lose their MHC I, leaving them vulnerable to NK cell killing. Once the decision is made to kill, the NK cell releases cytotoxic granules containing perforin and granzymes, which leads to lysis of the target cell.   Natural killer cells - figure 1
The genes for both MHC I and the NK cell inhibitory receptors which recognise them vary a lot between individuals. The versions (or alleles) of these genes a person carries have been linked to their ability to fight HIV infection and their risk of some autoimmune diseases. NK cell varieties also change with age and are affected by chronic viral infections such as cytomegalovirus (CMV).
Because of their ability to kill tumour cells, NK cells are an attractive target for cancer immunotherapies. Some therapeutic monoclonal antibodies rely on NK cell killing. Researchers are also developing treatments to activate NK cells using small molecules or cytokines, and even testing genetically modified living NK cells as therapies.

Reference

  • https://teachmephysiology.com/immune-system/cells-immune-system/t-cells/ 
  • https://www.immunology.org/public-information/bitesized-immunology/cells/natural-killer-cells

 

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