Effector Cell in the Immune System

Effector cells can be described as the cells of the immune system which have undergone the process of differentiation and maturation. These are the cells that mount specific responses in case of stimulus. The body’s immune system produces effector cells as a part of the immune response against pathogen or self cell, (in case of autoimmune disorders). 

Under pathogen attack, the immune cell undergoes differentiation, leading to the effector cell and memory cell production. In simpler terms, effector cells meaning are the immune cell that produces the response against the pathogen whereas memory cells are those that are retained in the body and divides only when there is a subsequent attack of the same pathogen. They are also used in immune effector cell therapy. 

Apoptotic, the major effector cell of the body includes effector T cell and, effector B cell. Other examples of immune effector cell include natural killer cell, these are the effector cell that plays a major role in the innate immune response of the body against the pathogen. Effector lymphocytes include effector T cell and effector B cell are the effector cells of adaptive immunity. On this page, the main focus will be on the generation and specific response of the immune effector cells.

Effector T Cell Generation

The process of generation of the effector T- cell is very complicated. To understand the generation and function of the immune effector cell, it is important to understand the production maturation and activation of the T-cell. T-cell produces specific effector function only after it has been activated, the unactivated T- cell is known as the naive T- cell. Upon activation T cells produce their effect by secreting biologically active compounds like cytokines, they also mediate cell cytotoxicity and promote the formation of plasma B cell (effector B-cell).

Production of T cell

T cells are produced in the bone marrow of the human body, they are produced from stem cells known as progenitor T-cell. These immature T-cells are known as thymocytes. Thymocytes lack both CD4 and, CD8. They are denoted as CD4- and CD8-. They’re the double negative. Thymocytes lack the following, CD4, CD8, TCR (T cell receptor). Thymocytes are marked by the presence of the following, c-Kit, CD17, CD44, CCR7, and CCR 9. These thymocytes then move to the thymus.

Maturation of T cell

T cell moves into the thymus for maturation, the movement is guided by the cytokine concentration gradient. The cortex of the thymus releases CCl 19, CCL21. Thymocytes have a receptor called CCR 7 which binds to the chemokine and results in chemotaxis. Double negative (DN-1) thymocytes move in the cortex of the thymus. It is important to note that DN-1 has to make physical contact with the cortical epithelial cell of the thymus. This cell has a factor called SCF (stem cell factor), which interacts with c-Kit and provides entry to the next phase of cell maturation. This interaction leads to the expression of the IL-7 receptor on the T cell.  IL-7 binds to its receptor which leads to an expression of CD25, at this stage, the thymocyte is known to be at DN 2 sate. This is the stage of cellular commitment, thymocytes that exist at this stage form gamma delta TCR and, the cell continues to develop normal alpha- beta TCR. 

IL-7 binding also initiates RAG 1 and RAG 2, recombination activating genes. They are responsible for TCR gene rearrangement. Once it begins, a beta chain of TCR appears marking the DN-3 stage. At the DN-4 stage, thymocytes lose their CD 25. Finally, CD 4 and CD 8 appears on the membrane marking it as double positive. 

It is important to note that T- cell must have self MHC restriction and can positively interact with the pathogen. For this, the double-positive cell undergoes positive and negative selection.

Positive Selection

Positive selection is done in the cortex region of the thymus. It refers to the selection of that cell that provides a positive response to self MHC, that is self MHC binding with the cortical epithelium cell. Or the cell can bind to the antigen-presenting cell, this phenomenon of binding is known as the chance binding. In case of binding with the MHC of either one, the unbound MHC is not further expression. This is the phenomenon of self MHC restriction. In case of no recognition of MHC, apoptotic signals are generated.

Negative Selection

It is done in the medulla region, in this case, cells that provide a negative response to the antigen or the only cell that do not interact are selected. This is done to ensure the minimum risk of autoimmune disorder. This is an important property of the effector cells of adaptive immunity. 

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T- Cell Activation

T cell activation refers to the interaction of the naive T cell with the antigen-presenting cells or immune effector cells, upon activation of naive cells undergo proliferation, differentiation, and apoptosis.  Proliferation refers to the clonal expansion of the cells, differentiation leads to the formation of the effector lymphocytes like immune effector cells and central memory t cells. Central memory t cells are the cell that acts during the second exposure of the pathogen. 

The Flowchart here Explains the Activation Process.

MHC- Ag complex binds to TCR

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The conformational change detected by LCK

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Phosphorylation of ITAM and binding of zap 70 to it.

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It leads to G- protein PLC signalling cascade

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The signalling results into the AP 1, NFAT, NFkB

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These are the transcriptional activators that result in the formation of effector t cells

central memory t cells.

It is important to note that for proper activation of effector cells of adaptive immunity, costimulatory signals are also required, CD80 of the APC interacts with CD28 of the effector T-cell. Improper activation due to lack of co-stimulatory signal is called clonal anergy

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Types of Effector Cell and Their Function

According to the effector cells definition, cells that provide specific immune response against the pathogen are termed as effector cells, T cell differentiates into TH1 TH2 and CTL cells are different subsets formed by differentiation of the effector T cell. Another class that differentiates from it is central memory t cell, they remain in the body for a long period and help in the differentiation of effector B lymphocytes during secondary infection. The major function of these subtypes is cytokine secretion.

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The Function of TH1 Subset of Effector Lymphocyte -

TH1 Subtypes Perform the Following Functions -

1. They play role in delayed hypersensitivity

2. They act in inflammation

3. They act in viral infections

4. They act in the differentiation of T cell into CTL by IL-2 production

5. They inhibit TH2 expansion by secreting interferon-gamma.

The Function of TH2 Subset-

TH2 Subset Perform the Following Functions -

1. The defining cytokine is the IL-2

2. Cytokine IL-4 and IL-5 secreted by their act in parasitic infection

3. They help in B-cell proliferation activation and, differentiation

4. They activate eosinophil.

5. They mediate IgM class switching to IgG

6. IL-4 and 110 act to suppress the clonal expansion of subset 1.

Cytokines Secreted by the Subset and Their Function

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Downregulation of Effector Cell

Downregulation refers to the control of effector function. The response must continue only to a limited amount of time, this is because excessive immune response leads to tissue damage and irregular metabolism depending upon the site of action. Effector cell has a limited life span when completed, they undergo apoptosis or cell death, cytokines secreted by them also play important role in feedback regulation.