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<p><font size="2"><strong>Helper T cells are T lymphocyte that belong to the CD4+ subset.<br />
</strong><br />
<strong>1. Types of Helper T Cells</strong><br />
- Th1 ; These participate in cell-mediated immunity. They are essential for controlling such intracellular pathogens as viruses and certain bacteria, e.g., Listeria and Mycobacterium tuberculosis (the bacillus that causes tb).<br />
- Th2 ; These provide help for B cells and, in so doing, are essential for antibody-mediated immunity. Antibodies are needed to control extracellular pathogens (which — unlike intracellular parasites — are exposed to antibodies in blood and other body fluids).<br />
<br />
<strong>2. Origin of Helper T Cells</strong><br />
Like all T cells, Th cells arise in the thymus. <br />
</font><font size="2">- When they acquire CD4, they are called pre-Th cells. <br />
- When they are presented with both an antigen and appropriate cytokines, they begin to proliferate and become activated. <br />
- It is the nature of the stimulation that determines which path they enter: the path leading to Th1 cells or the path leading to Th2 cells.<br />
<br />
The antigen-presenting cells (APCs) are called dendritic cells (DCs). There are several subsets of them: some produced by monocytes and others derived from a progenitor cell that gives rise to both them and monocytes. </font></p>
<p><font size="2"><strong>Dendritic cells </strong><br />
- ingest antigen by phagocytosis or pinocytosis; <br />
- degrade it in lysosomes, and <br />
- display fragments of the antigen at their surface for presentation to T cells. <br />
</font><font size="2">- secrete cytokines. <br />
(Dendritic cells can also present intact antigen directly to B cells. In this case, the engulfed antigen is not degraded in lysosomes but is returned to the cell surface for presentation to B cells bearing BCRs of the appropriate specificity.) </font></p>
<p><font size="2">There are at least two kinds of dendritic cell that present antigens to T cells: <br />
</font><font size="2">- DC1 and DC2 <br />
<br />
</font></p>
</strong><br />
<strong>1. Types of Helper T Cells</strong><br />
- Th1 ; These participate in cell-mediated immunity. They are essential for controlling such intracellular pathogens as viruses and certain bacteria, e.g., Listeria and Mycobacterium tuberculosis (the bacillus that causes tb).<br />
- Th2 ; These provide help for B cells and, in so doing, are essential for antibody-mediated immunity. Antibodies are needed to control extracellular pathogens (which — unlike intracellular parasites — are exposed to antibodies in blood and other body fluids).<br />
<br />
<strong>2. Origin of Helper T Cells</strong><br />
Like all T cells, Th cells arise in the thymus. <br />
</font><font size="2">- When they acquire CD4, they are called pre-Th cells. <br />
- When they are presented with both an antigen and appropriate cytokines, they begin to proliferate and become activated. <br />
- It is the nature of the stimulation that determines which path they enter: the path leading to Th1 cells or the path leading to Th2 cells.<br />
<br />
The antigen-presenting cells (APCs) are called dendritic cells (DCs). There are several subsets of them: some produced by monocytes and others derived from a progenitor cell that gives rise to both them and monocytes. </font></p>
<p><font size="2"><strong>Dendritic cells </strong><br />
- ingest antigen by phagocytosis or pinocytosis; <br />
- degrade it in lysosomes, and <br />
- display fragments of the antigen at their surface for presentation to T cells. <br />
</font><font size="2">- secrete cytokines. <br />
(Dendritic cells can also present intact antigen directly to B cells. In this case, the engulfed antigen is not degraded in lysosomes but is returned to the cell surface for presentation to B cells bearing BCRs of the appropriate specificity.) </font></p>
<p><font size="2">There are at least two kinds of dendritic cell that present antigens to T cells: <br />
</font><font size="2">- DC1 and DC2 <br />
<br />
</font></p>
