B- and T-cell memory

Your immune system is like the military - with two main branches, the innate immune response and the adaptive immune response.

Key features of the innate immune response are that the cells are non-specific, meaning that they don’t distinguish one invader from another invader, the response is really fast - occurring within minutes to hours, and there’s no memory associated with innate responses.

The adaptive response, which is mediated by lymphocytes like B and T cells - is the opposite of the innate immune response.

B and T cells have unique receptors - the B cell receptor and T cell receptor - that differentiate pathogens from each other using their unique parts - called antigens.

These receptors are developed while the T cell or B cell is developing in the bone marrow for B cells or thymus for T cells.

Once the cell has a unique antigen-specific receptor expressed on its surface it begins traveling through the lymphatic system - passing through lymph nodes in search for the one antigen that fits the receptor perfectly.

If they encounter that antigen, a signal gets delivered to the cell’s nucleus that lead to clonal expansion.

That’s where a single T cell or B cell replicates over and over - creating an army of clones that can combat the pathogen.

Once the immune response is complete, many of these cells die by apoptosis restoring the immune response to its original size - with one major change.

Some of the B and T cells become memory cells, which are basically a pool of lymphocytes that are all set to combat the pathogen, if they encounter it again!

Immunologic memory is sometimes referred to as a secondary or anamnestic response, and it’s different from the primary response.