This is a continuation of the previous post about the immune system, where now we focus on the lymphatic system.
Humans have a very advanced part of their immune system known as the adaptive immune system, which is only found in jawed vertebrates. In this system certain leukocytes, called Antigen-presenting cells (APCs), can bring in another line of defense by notifying T-cells that there is an infection.
How they accomplish this is quite remarkable: by tearing by apart the internalized pathogen and presenting the pathogen’s antigens on its surface. Antigens are structures which identify the pathogen and produce a specific immune response in T-cells. Our APC must now move to our T-cells, which are located in our lymph nodes, and present the antigens to activate them. The APC again uses chemotaxis to traverse through lymph vessels to reach the nodes. It presents the antigens attached to a special structure called MHC to “naive” T-cells.
T-Cells have special T-Cell Receptors (TCRs), which have constant parts which always bind to MHC, and variable parts which will only bind to specific antigens. Upon activation some T-cells become “T helper cells” (CD4+) and others become “Cytotoxic T-cells” (CD8+). The CD4+ cause the growth of more CD4+, which will release chemicals which stimulate more white blood cells to the site of the infection. Cytotoxic cell kill infected cells, as their name implies. They both return to the site of the infection through chemotaxis.
After the infection is eliminated, the specific antigens which the T-cells bound to will be “remembered” by memory T-cells. These cells bind only with the previous antigen and so the immune system is more prepared for a recurrence of the same infection.
Clearly I glossed over much, so read more for the real details. It’s an amazing system all accomplished with molecular machines!