Acquired Immunity:
I Picked It Up Along The Way

In simplest terms, acquired immunity is an immune response that we develop after birth in response to the introduction of new foreign invaders (pathogens, antigens, allergens, etc.)

It is activated by the innate immune system, also known as the inflammatory response.  As the inflammatory response winds down, the adaptive immunity takes care of what the innate response was unable to dispose of.

Three Responses We need to Know

Three separate responses make up acquired immunity and each will be covered in enough detail to understand the basics. 

In med school, the immunity course meets three days a week for a year and still can't keep up with all the new discoveries being made in how our immune system works.

The three acquired immunity responses are the humoral response which literally means "circulating in the blood"; the cell mediated response and the secretory phase. 

As a lead in to the humoral response we need to identify the various white blood cells. 

First off, there are Eosinophils, basophils and neutrophils.

These are vital to the innate or inflammatory response and are discussed in some detail in the Innate Immunity page. Then we have the small lymphocytes and monocytes.

For our purposes, it is the lymphocytes we are concerned with.

The lymphocytes are divided into T lymphocyte or just T-cells; and B lymphocytes or B-cells.  Both are produced in red bone marrow. 

B-cells leave the marrow as mature B cells; T-cells leave the marrow as "pre T-cells" and go to the Thymus gland to mature.  Once mature, T-cells leave the Thymus as mature T-cells.  Think of "B" for bone and "T" for Thymus. 

Next we need to know that "invaders" carry an Antigen which is complex molecules on the surface of the invader that allows them to be recognized by the body as non-self. 

Invaders can be pollen, food, drugs, virus, bacteria, transplanted tissue, venom, etc.  The important thing is that they are recognized by the immune system and trigger a response.

The first of the Adaptive Immunity Responses:
The Humoral Response

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The humoral involves B lymphocytes (B-cells). They make antibodies to bind to a specific antigen in a geometrically structured pattern designed to exactly match the antigen receptors. 

The common analogy is that of a lock and key.  It is very specific; each type of antibody is shaped to fit only one type of antigen. 

The way it works is that mature B-cells circulate in the blood and when called upon to meet a challenge, with the help of the T-cells (helper cells), they morph into plasma cells. 

The plasma cells are the ones that actually create the specific antibodies to bind to and inactivate the invading antigen in the body's fluids.
 
Plasma cells are short lived and die off when the threat is gone however there are always a few that survive and become memory cells.  These memory cells go dormant until that same invader is seen again. This feature is the essence of acquired immunity. 

At such time, they are activated immediately and the B-cells don't have to go through the step of creating memory cells all over again. 

The graphic below shows the action of first response to an antigen, the faster protection in a second exposure and the memory cells providing a very rapid response to a third exposure years later.


Illustration of first exposure and subsequent exposures showing action of memory cells in the amnestic response

The creation of the memory cells is called the inductive period and the speed at which the second exposure is dealt with is called the amnestic response. 

The beauty of acquired immunity is that, if we survive a first exposure, a second exposure at anytime in the future is dealt with much faster and efficiently than the first go-around.

Now let’s introduce a new word, Immunoglobulin (Ig).  It's just another name for antibody but there are five different types. Adaptive or acquired immunity uses them to deal with specific types of antigens (invaders) in our bodily fluids. 

Being humoral, they are in our circulatory system and are structurally large glycoproteins produced by the plasma cells mentioned above. 


Diagram:  The five immunoglobulin (antibody) configurations

Immunoglobulin configurations, created 7 Feb. 2006 by Martin Brändli (brandlee86)

The five immunoglobulins are:

  • IgG - comprises about 805 of the immunoglobulins and works to neutralize bacteria, virus, toxins, and crosses the placental membrane to give the fetus its immunity.
  • IgA - synthesized in the lining of the intestine.
  • IgD - a surface antibody that causes suppression, that is, tells the immune system when to back off, the fight is over. Thus it is an immuno-regulatory molecule.
  • IgE - takes care of allergies and parasites; these are the smallest in number.
  • IgM - has five branches and is confined to the blood stream. critical to fighting bacteria in babies, neutralization of toxins, promotes phagocytes, and up regulates the inflammatory response.
  • When it is said that the IgM promotes phagocytes and up regulates, it refers to phagocytosis.  A phagocyte is an immune cell that eats bacteria, breaks it apart, holds up the invaders antigen or "presents" it to a T-cell and enabling it to dock with the T-cell.

    The T-cell then stimulates the phagocyte to produce interleukin-1 (IL-1) which locks on to the T-cell and triggers its killing potential.

    Cell Mediated Response

    In the humoral response, we were mainly dealing with B-cell lymphocytes.  Now we move to the T-cell side of the acquired immunity equation.  It was stated earlier that T-cells mature in the Thymus then move into circulation.

    They carry surface antigen receptors with designations like CD8, CD4, etc. which define the programming that allows them to recognize a specific cell bound antigen. 

    By the way, the "CD" stands for "Cluster of Differentiation" and, so far, there are 320 proteins carrying a CD label. 

    They are not activated by circulating antigens, like the humoral B-cells, but need phagocytes to activate them by antigen presentation.

    Remember antigen presentation?  That's where the phagocyte chews up the invader and holds its antigen out to a T-cell to bind with.  There are two types of T-cells in acquired immunity.

    "Cytotoxic T-cells" become "killer T-cells" once activated and "Helper T-cells" activate both the humoral and cell mediated responses. 

    Killer T-cells are very effective in attacking body cells infected by a virus. 

    When activated, a cytotoxic T-cell attaches to an infected cell it has recognized and releases a substance called perforin; so named because it perforates or makes holes in the infected cell and destroys it.

    The humoral and cell mediated responses work hand in hand with helper T-cells as previously described.


    The Secretory Immune Response:
    The third type of
    acquired immunity response

    The defensive role of the secretory response in acquired immunity is similar to that of the skin except that it is present in all internal surfaces as opposed to the skin's external surfaces. 

    Lymph is the key word here.  The internal surfaces refer to lymphatic tissue of the intestines and the mucosa-associated lymphoid tissue
      
    Its job is to sit and wait for an invader.  Since the secretory response is available for immediate action, the objective is to dispose of the invader before it ever enters the body.  

    In one sense, it could be considered part of physical barrier since there are lymphatic elements in breast milk, tears, saliva, sweat.

    The ammunition that the secretory response uses is B-cell antibodies.

    Their abilities include:

  • preventing a local disease before it starts by using the antibody IgA to prevent attachment of the invader;
  • preventing penetration of mucosa thus avoiding a deeper infection;
  • and prevention of a carrier state in which the host is not sick but can pass on the disease. Typhoid Mary is the most often cited example but polio and hepatitis can also be passed on by "carriers".
  • In wrapping up our discussion, here is what the Merck Manual Home Edition has to say about Acquired Immunity just in case we missed something.

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