Lymphocytes are the cells of the immune system that play a crucial role in immunity. They present 20% to 40% in circulating white blood cells and 99% of cells in the lymph. There are three types of lymphocytes
- B lymphocytes (B cells)
- T lymphocytes (T cells)
- natural killer (NK) cells
B and T cells are small cells almost 8–10 microns in diameter having a large nucleus with dense hetero-chromatin. B and T cells are able to recognize and respond specifically to antigen.
They appear identical under a microscope. We differentiate B cells and T cells by signature of surface proteins that they express. Theses surface protein expressed by immune cells are often referred to by the cluster of differentiation (CD or cluster of designation).
Difference between B and T cells are given below in tabular form
|Sr#||Properties||B lymphocytes (B cells)||T lymphocytes (T cells)|
|1||Site of Maturation||B cells mature in Bone marrow||T cells mature in Thymus|
|2||Position||Mostly mature B cells occur outside the lymph node.||Mostly mature T cells occur inside the lymph node.|
|3||Membrane receptor||B cell receptor (BCR) for antigen||T cell receptor (TCR) for antigen|
|B cells have Shorter life span than T cells||T cells have large life span than B cells|
|5||Antigen Processing||B cells don’t need antigen processing to recognize the antigen||T cells require antigen processing through the Antigen presenting cells and require antigen complex with MHC|
|6||Cell Surface antibodies||Surface antibodies are present (IgM, IgD on B cells||Surface antibodies are absent|
|7||Abundance||They constitute about 20% of lymphocytes in blood||They constitute about 80% of lymphocytes in blood|
|8||Surface markers||B220 is first marker that appear during maturation of precursor B cells||Thy-1 is First marker of T cells.|
|9||Secretion||They secrete antibodies to encounter the antigen||T Cells secrete Lymphokines|
|10||Type of immunity||B cells involved in Humoral mediated immunity||T cells involved in Cell mediated immunity|
B lymphocytes (B cells)
The name B cell is derived from the its maturation site bursa of Fabricius in birds as bone marrow is its major site of maturation in humans, mice, and many other mammals. They display receptors that are B-cell receptor (BCR), a membrane-bound immunoglobulin (antibody) molecule that binds to antigen. B220 (CD45) is the marker that design first on the precursor B cell and remain throughout the life span of B cells. Mature B cells express receptors for the complement such as
- CR1 (CD35) binds to the C3b
- CR2 (CD21) binds to the C3d
Function of B lymphocytes
B cells are the cells of immune system that produce antibodies and are center of the adaptive humoral immune system. B cells bind with the antigen to inactivate or kill the antigen. B lymphocytes bind to antigen through a process known as somatic hypermutation to improve its ability of binding. They produce antibodies of several different functional classes through a process known as class switching.
B cells differentiate into two type of cells that are
- Plasma cells: B cells differentiate into effector cells called plasma cells that make large amount of antibodies. They lose expression of surface immunoglobulin its main difference from the B cells. A single plasma cells have ability to secrete from a few hundred to more than a thousand molecules of antibody per second. They secrete one of the five classes of antibody IgM, IgG, IgA, IgD and IgE.
- Memory cells: They don’t secret antibodies but are cells that respond more rapidly and effectively to pathogens that have been encountered previously.
T lymphocytes (T Cells)
T lymphocytes name derive from its maturation site which is thymus. The membrane receptors for antigen called T cell receptors (TCR). T cells receptors only recognize antigen when it is processed by antigen presenting cells (APS) and bound by the Major Histocompatibility complex (MHC). The first marker of t cells is Thy-1 appear during the maturation of T cells. Other markers of T cells are CD4 and CD8.
Types of T cells
T cells are divided into Two major cells
- T helper (TH) cells
- T cytotoxic (TC) Cells
T helper (TH) cells: T cells that display CD4 cells function as TH cells and they recognize antigen complex with MHC class II. After binding with antigen complex with MHC II they become active, proliferate and differentiate into effector T cell subset
- T helper type 1 (TH 1) cells: TH 1 regulate immune response against the intracellular pathogen.
- T helper type 2 (TH 2) cells: TH 2 regulate immune response against the extracellular pathogen.
- T helper type 17 (TH 17) cells: TH 17 helps in defense against the fungi and secrete the IL-17.
- T Follicular helper cells (TFH): TFH regulates B cells development in germinal center and play important role in humoral immunity.
T cytotoxic (TC) Cells: T cells that display CD8 cells function as TC cells and they recognize antigen complex with MHC class I. They have a vital function in monitoring the cells of body. They eliminate the cells that display foreign antigen complex with MHC class I.
Another type of CD4 T cells is Regulatory T cell (TREG). They express CD4 and CD25 on their surface that help in their identification.