Adaptive Immunity

Adaptive immunity is part of the immune system that can provide long-lasting immunity. It’s sometimes called acquired immunity because it’s not present at birth and develops over time.

Now, adaptive immunity has certain characteristics that sets it apart from the innate, or the general, immune defense an individual is born with, including that it’s inducible, specific, systemic, long-lived, and it has an immunologic memory.

So, inducible and specific mean that adaptive immunity is not always actively present, but instead, it's explicitly developed against a particular antigen, which are molecules found on the surfaces of microorganisms and abnormal cells that trigger an immune response.

Next, systemic means that immune cells can mount an attack throughout the body.

Finally, adaptive immunity is long-lived and has an immunologic memory, meaning certain immune cells called memory cells, linger in the body, so the next time the body is exposed to this same pathogen, it can mount a faster and stronger response.

The main components of adaptive immunity include lymphocytes, or white blood cells, and antibodies. The most prominent lymphocytes in the adaptive immune system are T lymphocytes, or T cells, and B lymphocytes, or B cells. These cells interact with other components of the immune system, such as dendritic cells, natural killer cells, and macrophages. Antibodies, also called immunoglobulins, are glycoproteins that assist in fighting infections.

Now, the main mechanisms of adaptive immunity are induction, cell-mediated immunity, humoral immunity, and immunologic memory.

Induction involves the activation of the adaptive immune system in response to an antigen, and the process differs between B and T cells.

The first step in T cell induction is when certain cells, most often macrophages and dendritic cells, serve as antigen-presenting cells, or APCs, because they identify, process, and present antigens to a T cell, which is called a naïve T cell at this stage. However, the T cell will only recognize the antigen if it’s presented on a molecule called a major histocompatibility complex, or MHC, which are proteins found on the surface of most human cells.

Now, there are two main types of MHC proteins that work with T cells. MHC I proteins present to T cells that express CD8 surface receptors. Once the antigen is presented, CD8 T cells are activated and differentiate into cytotoxic T cells, or Tc cells, which kill infected or other abnormal cells like cancer cells.