Macrophages and Antigens
When non-self antigens enter the blood or tissue fluid, infection begins. A non-self antigen can range from a virus or bacterial cell to a transplanted cell.
In order for an immune response to be activated the foreign antigen must make itself known to antigen-presenting cells. Many cells have this role but the best are the macrophages that can ingest the cells by phagocytosis. A few of the antigens from the foreign cell appear on the surface of the macrophage, turning it into an antigen-presenting cell. This way, the foreign cell is amplified without harming the body further by actually increasing the number of pathogens present. Macrophages also secrete chemicals which activate the next phase: clonal selection.
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Clonal selection
Antigen-presenting cells mix with helper T-cells in the blood until they find one which carries a matching receptor molecule to the antigen it’s carrying. The tight binding activates the helper T-cell to produce cytokines. Cytokines are chemicals which stimulate immature B and T lymphocytes to start proliferating and differentiating. They divide by mitosis and in the end about 106 cloned lymphocytes are produced. This is known as clonal selection. All these B and T lymphocytes have the binding site for the foreign antigen and are now equipped to kill the foreign body.
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Cellular immunity
The stimulated T-lymphocytes differentiate into cytotoxic T-cells, otherwise known as killer T-cells. They bind to the pathogen’s antigens and create pores in their cell membrane. This allows water to flow in causing the cell to burst, a process known as cell lysis.
The T-lymphocytes kill the foreign cell directly and so this is called cellular immunity.