Cells of the Adaptive Immune System
Cells of the Adaptive Immune System
- The lymphocyte is a key cell of the Adaptive Immune System .
- Lymphocytes make up 20% to 40% of all circulating White Blood Cells.
- A typical small lymphocyte is 7-10 m in diameter and has a large rounded nucleus that may be slightly indented. The nuclear chromatin is dense and has a dark blue stain. Cytoplasm is sparse, with few organelles and no specific granules, and is surrounded by a narrow ring.
- The cytoplasm is a lighter blue. These cells are distinct because they develop from an Hematopoietic stem cells (HSCs) and then differentiate in the primary lymphoid organs, namely the bone marrow and the thymus.
Lymphocytes are classified into three types:
- B cells
- T cells
- and natural killer (NK) cells
This is based on specific functions and proteins found on their cell surfaces. Adults’ peripheral blood lymphocytes contain 10% to 20% B cells, 61% to 80% T cells, and 10% to 15% NK cells.
The three types of cells are difficult to distinguish visually.
1. B Cells
- B cells are derived from a lymphoid precursor, which differentiates into a T cell, B cell, or NK cell depending on cytokine exposure. B cells remain in the environment provided by bone marrow stromal cells.
- B-cell precursors go through a developmental process that prepares them for their role in antibody production while also limiting the types of antigens to which any given cell can respond.
- As a result, a B lymphocyte is programmed to produce a unique antibody molecule. B cells can be identified by the presence of membrane-bound antibodies of two types, Immunoglobulin M (IgM) and Immunoglobulin E (IgE) (IgD).
- Other surface proteins found on B cells include CD19, CD21, and class II major histocompatibility complex (MHC) molecules.
2. T Cells
- T cells are so named because they develop in the thymus.
- Thymocytes, or lymphocyte precursors, enter the thymus from the bone marrow via the bloodstream. T cells develop distinct surface markers that allow them to recognize foreign antigens bound to cell membrane proteins known as MHC molecules.
- T cells contribute to immunity by stimulating B cells to produce antibodies, assisting in the killing of tumor cells or infected target cells, and helping to regulate both the innate and adaptive immune response. Cell mediated immunity is the name given to this process.
T cells are classified into three types based on their functions:
- Cytolytic and
- Regulatory T cells.
The presence of the CD3 marker, as well as CD4, or CD8, on the cell surface distinguishes the subtypes.
T cells that express the CD4 receptor are primarily helper or regulatory cells, whereas CD8-positive (CD8+) T cells are cytotoxic T cells. In peripheral blood, the ratio of CD4+ to CD8+ cells is approximately 2:1.
- Refer to differentiation clusters.
- Is the most common classification of T-lymphocyte, referring to those that carry CD4 antigens; the majority are helper cells.
- Is the most common classification of T lymphocytes, and include cytotoxic T lymphocytes and suppressor cells.
CD4 on T-helper Cells
- T helper cells have a protein on their surface called CD4.
- In order to infect a cell, HIV binds to the CD4 protein.
- As an HIV infection worsens, the number of cells expressing CD4 decreases.
- Thus, knowing the number of CD4 cells in an HIV-infected individual provides an estimate of the severity of his/her disease.
Also read: Cells of the Innate Immune System
3. Natural Killer (NK) Cells
- Natural killer (NK) cells are so named because they have the ability to kill target cells without prior exposure to them.
- A small percentage of lymphocytes do not express T or B cell markers.
- NK cells do not appear to mature in the thymus and instead appear to mature in the bone marrow.
- NK cells are larger than T and B cells, measuring about 15 m in diameter, and have kidney-shaped nuclei with condensed chromatin and prominent nucleoli.
- NK cells, also known as large granular lymphocytes, account for 10% to 15% of the circulating lymphoid pool and are found primarily in the liver, spleen, and peripheral blood.
- NK cells do not have unique surface markers, but they do express a specific combination of antigens that can be used to identify them. CD16 and CD56 are two examples of such antigens.
- CD16 is a receptor for the nonspecific end of antibodies.
- Because of the presence of CD16, NK cells can make contact with and then lyse any cell coated with antibodies. NK cells are also capable of recognizing any foreign cell and serve as the first line of defense against virally infected cells and tumor cells.
- Although NK cells have traditionally been thought to be part of the innate immune system because they can respond to a wide range of antigens, it now appears that they, like T cells, can develop memory to specific antigens.
- Normally, NK cells have a half-life of 7 to 10 days, but new evidence suggests that they can survive for longer periods of time due to their ability to generate highly specific memory cells.
- As a result, they play an important role as a transitional cell between the innate and adaptive immune responses to pathogens.