Policemen of our body

Just like policemen save us from criminals, immune cells save our body!

T cell
There are 2 groups of policemen I'd like to drag your attention to - Lymphocytes and antigen presenting cells.

Lymphocytes are one of the many types of WBCs’ produced in bone marrow by the process of haematopoiesis.

There are 2 types of lymphocytes: B-lymphocytes and T-lymphocytes.

B-lymphocytes have a membrane bound antibody molecule. When a naïve B-cell first encounters an antigen that matches its membrane bound antibody, the binding of antigen to the antibody causes a cell to divide rapidly into memory B-cells and effector B-cells also known as plasma cells.

Memory B-cells survive for a longer time than the naive B-cells and have same antibody molecule on its surface as that of the naive cell. This memory B-cell then helps our body to recognize the antigen if it ever enters our body again and helps in destroying the same!

Plasma cell secretes antibodies and has a lesser life span. Even if plasma cells have a lesser life span they secrete enormous amounts of antibodies.

T-lymphocytes have a different way of encountering an antigen! Here’s a small story which will help you understand how exactly a T-cell encounters an antigen.

A local police encounters a most wanted criminal and puts him behind bars. The criminals' information is now sent to the FBI for further investigation. It is the FBI which will take appropriate actions against the criminal.

Now, imagine the criminal is the antigen which has been encountered by a cell - local police. The cell represents a part of antigen - criminals information on their cell surface so that the T-cells - the FBI can identify the antigen and destroy it!

T-cells can bind to only those antigens which are present on cell membrane proteins called Major histocompatibility complexes. The FBI can trust the police only if they come with a badge.

There are 2 class of these MHC Complexes :
• Class I (present on the surface of nucleated cells)
• Class II (present on the surface of Antigen Presenting Cells)

Also, there are 2 major types of T-cells: T-Helper cells and T-Cytotoxic cells.

The TH-cells recognizes antigens on MHC Class II Complex present only on Antigen Presenting Cells and TC-cells cells recognizes antigens on MHC Class I Complex present only on Nucleated cells.

T cells in action!

TH-cell when interacts with an antigen present on MHC Class II Complex the cell is activated and becomes an effector cell which secretes cytokines.

Cytokines secreted by TH-cells plays an important role in activation of activating:
• B-cells
• TC-cells
• Macrophages
• Various other cells that are involved in immune response

TC-cell proliferates under the action of Cytokines and differentiates into an effector cell called the Cytotoxic T lymphocyte (CTL) it does not secretes cytokines as that of TH-cells where as it has got a cell killing ability (cytotoxic activity).

The CTL helps in monitoring the self-cells (nucleated) which have been infected by any virus or bacteria that displays the antigen.

Antigen presenting cells are specialised cells which include macrophages, B-lymphocytes and dendritic cells.  These cells internalise antigens either by phagocytosis or by endocytosis and then display part of the antigen on their membrane bound MHC class II Complex thus allowing the TH-cells to recognize them and activate an immune response.

Written by Komal M. Kadam

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