T Lymphocyte Function

Antigen presenting cells or APCs are B cells, dendritic cells, or macrophages that can activate the T cells by presenting antigens on their surfaces. Remember that macrophages and dendritic cells participate in phagocytosis, while B cells mainly make antibodies. B cells also present antigens to T cells. All APCs will have MHC II molecules on their surfaces. They ingest antigens and spit them back out on their surface to present to other immune cells.

Dendritic cells are classic APCs. They recognize a pathogen and attach to it, eventually internalizing it. There are lysosomes that digest most of the pathogen, except for parts that are separated by proteases in order to become epitopes that will cause an immune response to happen. Only selected epitopes are presented by the APCs. They ultimately get attached to the MHC II molecules, which is where the presentation happens.

MHC I molecules are found on all nucleated cells, presuming they are healthy cells. NK cells and other immune cells recognize the MHC I molecules, leaving the cells with these molecules on them alone. Infected cells are associated with pathogen-specific antigens attached to their MHC I molecule, which marks them for destruction.

T LYMPHOCYTE FUNCTION

Humoral immunity is associated with mainly extracellular pathogens. Cells that are already infected with a pathogen need to be gotten rid of by T cells. T cells are responsible for cell-mediated immunity. T cells are made in the bone marrow but mature in the thymus, where they are referred to as thymocytes before they become mature.

T cells need to be properly selected by the thymus before they can be released. This is called thymic selection. The first step involves the making of T-cell receptors necessary for APC activation. Those that are defective and do not make these receptors are killed through apoptosis. This is called negative selection. Next, there is positive selection. Those cells that do not interact with the body’s MHC molecules are selected out. The third step involves removing cells that react too much to the self MHC molecules. This prevents things like autoimmunity, which involves accidentally recognizing the self-cells as being foreign. This is also referred to as central tolerance.

If central tolerance fails, there is a process called peripheral tolerance. This also prevents autoimmune diseases. It involves blocking the activity of self-reacting T cells by cells called regulatory T cells. They block the activity of these reactive T cells and make anti-inflammatory cytokines. After thymic selection, only 2 percent of thymocytes persist and these become mature naïve T cells until they can be activated.

There are actually several types of T cells. Helper T cells or CD4 cells, cytotoxic T cells or CD8 cells, and regulatory T cells. The CD4 cells and CD8 cells are called this because they make certain kinds of cluster of differentiation or CD molecules. Helper and regulatory T cells can only be activated by antigen presenting cells, while cytotoxic T cells will respond to any MHC I cell or basically any nucleated cell. Helper T cells augment the function of cytotoxic T cells and cytotoxic T cells do the actual cell killing. A cell that participates in cell killing is called an effector cell.

There are specialized T cell receptors on the different T cells. These are related to antibodies like IgD and IgM and also have a variable and constant region. They are actually smaller than antibodies, however, and are less complex than antibodies, made of two peptide chains. Each T cell receptor binds to a specific epitope. Genetic rearrangement in the T cell allows for millions of different and unique receptors on the T cells.

When a helper T cell gets activated, it can form four different types of cells. There are T helper 1 cells and T helper 2 cells as well as memory helper T cells. The first two types of helper cells do not live long and are involved in immediate immune activities. Memory T cells, however, live a long time in order to remember a specific epitope. Helper T 1 cells make cytokines and work in all aspects of the immune system. Helper T 2 cells act in the humoral immune system by activating B cells and by directing B cell maturation and antibody production. There is a third helper cell type called helper T 17 cells. A lack of these cells causes chronic mucocutaneous infections.

Cytotoxic or CD8 cells respond to any type of cell that makes an MHC molecule. They differentiate and clone themselves after recognizing a cell as being foreign or infected. These are the cells that act in similar ways to NK cells by killing infected cells. Like NK cells, they make perforin and granzymes that can induce cell apoptosis. The major