Article 2 covered the innate immune system, the boy’s frontline defence. Now we move onto the immune system’s memory. The adaptive (acquired) response is the second line of attack. It takes longer to organise and initiate (kicking in at around day 4-7) but its attack is targeted to the specific pathogen and acts as a long lasting memory. If the same pathogen is identified a second time, the immune response can be quicker and in some cases, “immunity” will have been gained so the pathogen will have no clinical consequence. The adaptive response is different depending on if the pathogen is in the blood and fluids (antibodies are made) or inside the tissue (a cell mediated response is necessary). T cells are responsible for the cell-mediated response. They recognise infected cells and destroy them.
This attack is specific and they can respond selectively to many non-self materials. The T cells are formed in the bone marrow and then migrate to the Thymus (hence T tells) where they start to express (make) molecules on their surfaces termed ‘receptors.’ They all express T cell receptors (TCR) and then either CD4 or CD8. So, each T cell with ether has TCR + CD4 or TCR + CD8. This duo of receptors on the cell’s surface is the lock. The key is found on the surface of the Antigen Presenting Cells (we talked about these in innate immunity), in the form of their own surface proteins (MHC I or II). The APC, if you remember from before, has engulfed the pathogen and then taken a little bit of it to its surface and attached it to its MHC to “present” to the T cells. Once the APC’s key fits in the T cell’s lock (a very specific pairing), the T cell is activated to multiply into one of 3 specialised T cells that will ultimately work together to kill the infection. Helper T cells express CD4 (and bind to MHC II) and they HELP to activate other immune cells. Cytotoxic T cells express CD8 (and mind to MHC I) and they kill the pathogens or infected cells. Regulatory T cells express CD4 and CD25 and they make sure the immune system targets only foreign material and not self. This reduces autoimmune diseases.
B cells don’t kill pathogens themselves but are responsible for creating antibodies and thus the immunological memory.
They are also created in the bone marrow and are then deployed to the lymphatic system of the body when they have matured.
Here, mature but naive B cells that have produced multiple antibodies bound to their surface encounter an antigen that fits one of their locks, and they mature.
It will then quickly divide into either memory B cells, or Plasma B cells. Memory B cells will express the same membrane bound antigen as its parent B cell the one that is divided to make it). They can survive for decades and act quickly when a previously known pathogen returns. When they reencounter the antigen, they then differentiate into Plasma B cells (they do this faster than the naïve cells did). Plasma B cells will produce the same antibody as its parent B cell too but instead of being membrane bound, they are secreted (released) into the bloodstream. These can then be deployed around the body to detect any residual pathogen with the antigen fitting the antibody. If you have any questions on COVID-19 and/or our immune system, send them over to me on my instagram @drsteph_scrubsvsshreds and I will do my very best.
Dr Stephanie Hill
BSc, MBBS
