Question

1. Path selected : Pluripotent stem cell -----> Lymphoid stem cell -----> Thymus ------> T-lymphocytes (T-cells)
   
2. trace the path, and describe where the cell is formed and which system(s) it is typically found in when it is mature.
3. structural description of resulting cell
4. function of resulting cell
   • is it involved in innate or adaptive immunity?
   • what line of defense does it function in: first, second or third?
   • what is its role in immunity?

provide any source if used

Answer 1

This is a photo of a lymphocyte in a blood smear. Most of the lymphocytes are small; a bit bigger than red blood cells, at about 6-9µm in diameter.

The rest (around 10%) are larger, about 10-14µm in diameter. These larger cells have more cytoplasm, more free ribosomes and mitochondria. Lymphocytes can look like monocytes, except that lymphocytes do not have a kidney-bean shaped shaped nucleus, and lymphocytes are usually smaller. Larger lymphocytes are commonly activated lymphocytes.They have a small spherical nucleus and has abundant dark staining condensed chromatin. Not much cytoplasm can be seen, and it is basophilic (pale blue/purple staining).

T cells are a type of lymphocyte, or white blood cell, that play an important role in the adaptive immune system.

T cells develop from hematopoietic stem cells in the bone marrow. Progenitors of those cells migrate to the thymus, here they are known as thymocytes. Thymocytes mature in a series of steps based on development of cell surface markers. Most cells in the thymus develop into ɑβ T cells, while about 5% become γδ T cells.

Immature T cells do not express either the CD4 or CD8 antigen. They are known as double-negative (DN) cells (CD4-CD8-). Through the process of development, they become double-positive cells (CD4+CD8+), then mature into single-positive (CD4+CD8- or CD4-CD8+) thymocytes and are released to peripheral tissues.

Most thymocytes die in the process of development. The remaining 2% become mature T cells.

Once they have completed their development in the thymus, T cells enter the bloodstream and are carried by the circulation. On reaching a peripheral lymphoid organ they leave the blood again to migrate through the lymphoid tissue, returning to the bloodstream to recirculate between blood and peripheral lymphoid tissue until they encounter their specific antigen. Mature recirculating T cells that have not yet encountered their antigens are known as naive T cells. To participate in an adaptive immune response, a naive T cell must first encounter antigen, and then be induced to proliferate and differentiate into cells capable of contributing to the removal of the antigen. We will term such cells armed effector T cells because they act very rapidly when they encounter their specific antigen on other cells. The cells on which armed effector T cells act will be referred to as their target cells.

T cells originate in the bone marrow and mature in the thymus. In the thymus, T cells multiply and differentiate into helper, regulatory, or cytotoxic T cells or become memory T cells.

There are several types of T cells. Effector T cells include the categories of helper, killer, memory, and regulatory T cells. Helper T cells recruit other lymphocytes in an immune response. Cytotoxic, or killer, T cells destroy infected cells.

Memory T cells are long-lived and can expand quickly to large numbers of effector T cells if they are re-exposed to their cognate antigen. Regulatory T cells maintain immunological tolerance by shutting down T cell-mediated immunity when an immune response is complete and suppressing autoreactive T cells.

Which is the site of T cell maturation and selection?

thymus: A ductless gland consisting mainly of primary lymphatic tissue that is the site of lymphocyte maturation and selection. Immature T lymphocytes are produced in the red bone marrow and travel to the thymus for maturation. Thymic selection is a three-step process of negative and positive selection that determines which T cells will mature and exit the thymus into the peripheral bloodstream.

Where are T cells stored in the body?

Memory T cells are stored in the lymph nodes and spleen and may provide lifetime protection against a specific antigen in some cases.

Basic Structure of the T-cell receptor

T-cell receptors consist of two polypeptide chains. A typical T cell may have as many as 20,000 receptor molecules on its membrane surface

What is the function of T cells?

T cells (also called T lymphocytes) are one of the major components of the adaptive immune system. Their roles include directly killing infected host cells, activating other immune cells, producing cytokines and regulating the immunity.

There are 3 main types of T cells: cytotoxic, helper, and regulatory. Each of them has a different role in the immune response.When helper T cells recognize a peptide on an antigen presenting cell, they become activated and begin to produce molecules called cytokines that signal to other immune cells.

Helper T cells become activated when they are presented with peptide antigens by MHC class II molecules, which are expressed on the surface of antigen-presenting cells (APCs). Once activated, they divide rapidly and secrete cytokines that regulate or assist the immune response.

T cells are involved in adaptive immunity.

T cells are involved in third line of defence. Our third line of defense is specific immune responses - T Cells and B Cells. There are many types of each which work like a close knit team to destroy pathogens

The first line of defence (or outside defence system) includes physical and chemical barriers that are always ready and prepared to defend the body from infection. These include your skin, tears, mucus, cilia, stomach acid, urine flow, 'friendly' bacteria and white blood cells called neutrophils.

The second line of defense is nonspecific resistance that destroys invaders in a generalized way without targeting specific individuals: Phagocytic cells ingest and destroy all microbes that pass into body tissues. For example macrophages are cells derived from monocytes (a type of white blood cell)

The Innate vs. Adaptive Immune Response

	Line of Defense	Cells
Innate (non-specific)	First	Natural killer cells, macrophages, neutrophils, dendritic cells, mast cells, basophils, eosinophils
Adaptive (specific)	Second	T and B lymphocytes

Cellular immunity is mediated by T lymphocytes, also called T cells. Their name refers to the organ from which they’re produced: the thymus. This type of immunity promotes the destruction of microbes residing in phagocytes, or the killing of infected cells to eliminate reservoirs of infection. T cells do not produce antibody molecules. They have antigen receptors that are structurally related to antibodies. These structures help recognize antigens only in the form of peptides displayed on the surface of antigen-presenting cells.

T cells consist of functionally distinct populations. These include naive T cells that recognize antigens and are activated in peripheral lymphoid organs. This activation results in the expansion of the antigen-specific lymphocyte pool and the differentiation of these cells into effector and memory cells. Effector cells include helper T cells, and cytolytic or cytotoxic T cells. In response to antigenic stimulation, helper T cells (characterized by the expression of CD4 marker on their surface) secrete proteins called cytokines, whose function is to stimulate the proliferation and differentiation of the T cells themselves, as well as other cells, including B cells, macrophages, and other leukocytes. Cytolytic or cytotoxic T cells (characterized by the expression of CD8 marker on their surface) kill cells that produce foreign antigens, such as cells infected by viruses and other intracellular microbes.

Cell-mediated immunity: T cells promote the killing of cells that have ingested microorganisms and present foreign antigens on their surface.

Memory T cells are an expanded population of T cells specific for antigens that can respond rapidly to subsequent encounter with that antigen and differentiate into effector cell to eliminate the antigen. Another class of T cells called regulatory T cells function to inhibit immune response and resolve inflammation. Their major role is to shut down T cell-mediated immunity toward the end of an immune reaction.

Cell-mediated immunity involves cytotoxic T cells recognizing infected cells and bringing about their destruction.

Sources:https://proteinswebteam.github.io/interpro-blog//potm/2005_3/Page1_files/image006.gif

https://courses.lumenlearning.com/boundless-microbiology/chapter/t-cells-and-cellular-immunity/

https://www.ncbi.nlm.nih.gov/books/NBK10762/(source)