Cells of immune system involved in adaptive immunity.

Lymphocytes are the central cells of adaptive immunity that bearing antigen receptors.

Lymphocytes constitute, 20% to 40% of the body’s white blood cells and 99% of the cell in the lymph.

Lymphocytes division by function and cell membrane components,

The lymphocytes can be subdivided into three populations types:-

  1. B cells
  2. T cells
  3. Natural killer cells

B-cells and T-cell have their own distinctive family of antigen receptors and natural killer cells (NK) are large, granular lymphocytes that are part of the innate immune and NK cells don’t express the surface markers that characterize B or T cells.

T cells that have not interacted with antigen are referred to an as naive cell. Naive cells are small, motile, nonphagocytic cells that cannot be distinguished from each other morphologically.In their unactivated state, they remain in the Gphase of the cell cycle and they are known as small lymphocytes (6 μm in diameter).

Under appropriate conditions,

The interaction of small lymphocytes with the antigen induces these cells to progress through the cell cycle from G0 into G1 and subsequently into S, G2, and M. As the cell cycle proceeds, lymphocytes enlarge into 15 μm-diameter cells called lymphoblasts.

The lymphoblasts have a higher cytoplasm to nucleus ratio and they proliferate and eventually differentiate into effector cells or into memory cells.

The antibody secreting effector cells of the B-cell lineage are called Plasma cells.

The effector cells of the T-cell lineage include the cytokine-secreting T helper cell (TH cell) and  T cytotoxic lymphocyte (Tc cell) lineage known as CTLs (cytotoxic T lymphocyte).

Some progeny of B and T lymphoblasts differentiate into memory cells. The persistence of the memory cells provides lifelong immunity to many pathogens.Memory cells look like small lymphocytes but they the presence or absence of certain molecules on their cell membranes distinguish them from naive cells.

Different lineages of lymphocytes can be distinguished by their expression of membrane molecules recognized by particular antibodies. The monoclonal antibodies that react with a specific membrane molecule are grouped together as the cluster of differentiation (CD).

Some general characteristics and functions of B and T lymphocytes

B lymphocytes ( B cells )

When a naive B cell counters the antigen that complements its membrane bound antibody, it starts dividing rapidly and its progeny differentiates into memory cells and into effector cells( called plasma cells).

The memory cell expresses the same membrane bound antibody as their parent B cell and have a longer lifespan than naive cells while the plasma cell produces the antibody that can be secreted and have little or no membrane bound antibody.Plasma cells are end stage cells and do not divide.

T lymphocytes ( T cells )

The T-cell mature in the thymus and they express a unique antigen binding molecule on its membrane called the T-cell receptor. T-cell receptors only recognize the antigen that is bound to cell membrane proteins called major histocompatibility complex (MHC) molecule unlike the membrane bound antibodies on B cells, which can recognize antigen alone.

There are tow major types of MHC molecules:-

  1. Class I MHC – Expressed by nearly a nucleated cell of vertebrate species.
  2. Class II MHC – Expressed by only a few cell types that are specialized for antigen presentation.

when a T cell recognizes antigen, it proliferates and differentiates into various effector T cells and memory T cells.

The subpopulations of T cell are:-

  1. T helper (TH) cells – display CD marker CD4.
  2. T cytotoxic (TC) cells – display CD marker CD8.
  3. T regulatory (T reg ) cells – Identified by the presence of both CD4 and CD25.

Following activation by interaction with appropriate antigen-MHC complexes, T cells differentiate into effector cell ( instead of effector cell some differentiate in memory cells), Tcells, macrophages and various other cells that participate in the immune response.

Recognition of antigen-MHC complexes by a TC triggers its proliferation and differentiation into an effector cell called a CTL(cytotoxic T lymphocyte) or into memory cells. The cytotoxic T lymphocytes have a vital function in monitoring the cell of the body. They also eliminate the cell that displays foreign antigen complexed with class MHC, such as virus-infected cells, tumor cells, and cell of a foreign tissue graft.

Natural killer cells (NK)

Natural killer cells are large, granular lymphocytes that display cytotoxic activity against a wide range of tumor cells and against cell infected with some but not all viruses. They can recognize tumor or virus-infected cells despite lacking antigen specific receptor. NK cells are the part of innate immunity.

For the recognition their target cells, NK cell employs NK-cell receptors to distinguish abnormalities, like reduction in the overall display of class I MHC molecules and cell infected by some viruses.

because,

The Natural Killer cells express a membrane receptor (CD16) for a specific region of the antibody molecule, they can attach to these antibodies and subsequently destroy the targeted cells. This is an example of a process known as antibody-dependent cell mediated cytotoxicity (ADCC).

Granulocytic cells

The granulocytes ar classified as

  1. NeutrophilsPhagocytic
  2. EosinophilsPhagocytic
  3. BasophilsNon-phagocytic

Neutrophils (multilobed nucleus and a granulated cytoplasm)

The neutrophils are often called Polymorphonuclear (PMN).Generally, they are the first to arrive at a site of inflammation. This temporary increase in circulating neutrophils, called leukocytosis.This increase in neutrophils is used medically as an indication of infection.Movement of circulating neutrophils into tissues called extravasation.

Steps in extravasation:-

  1. Initially, the cell adheres to the vascular endothelium.
  2. Secondly, they penetrate the gap between adjacent endothelial cells lining the vessel wall.
  3. At last, they penetrate the vascular basement membrane, moving out into the tissue spaces.

Eosinophils

The eosinophil has a bilobed nucleus and a granulated cytoplasm that stains with the acid dye, eosin red. Eosinophils are motile phagocytic cells (similar to neutrophils) that can migrate from the blood into the tissue spaces. Eosinophil involves in the defense against parasitic organisms by secreting the contents of eosinophilic granules, which may damage the parasite membrane.

Basophils (a lobed nucleus and heavily granulated cytoplasm)

The basophil stains with the basic dye methylene blue. From their cytoplasm, Basophils release pharmacologically active substances which play a major role in certain allergic responses.

Mast cells

Mast cell precursors are released into the blood as undifferentiated cells. They do not differentiate until they leave the blood and enter the tissues. They are present in the skin, connective tissues of various organs, and mucosal epithelial tissue of the respiratory, genitourinary, and digestive tracts.

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