Acquired Immunity: The body defense mechanism against specific antigens

Acquired immunity is also known as adaptive or specific immunity. As the name specifies, it is capable of recognizing and selectively eliminating specific pathogens. Acquired immunity is a characteristic of vertebrates only. This kind of defense system is triggered in response to an exposure to a microorganism. However, specific defense mechanisms require several days to be activated. Following are some of the acquired immunity features:

Specificity: The immune system is capable of distinguishing various foreign molecules.

Diversity: It has the ability to recognize and distinguish a vast variety of foreign molecules.

Discrimination between self and non-self: It is able to recognize molecules that are foreign or non-self. In addition, it also recognizes self-bodies and does not harm them.

Memory: When the immune system first encounters a microbe or any foreign agent, it generates an immune response and finally eliminates the invader. The immune system retains the memory of its encounter with a particular pathogen. So, when the same pathogen comes in contact with the immune system, it recognizes it and evokes a heightened immune response.

Specific immunity is associated with two major groups of cells, which are lymphocytes and antigen-presenting cells. A healthy human body possess about a trillion lymphocytes. The lymphocytes are categorized into two types, which are T lymphocytes or T cells and B-lymphocytes or B cells. All these lymphocytes are produced in the bone marrow and the process of their production is called haematopoiesis.

Both B and T cells together generate two kinds of specific immunity in the body.

1. Cell mediated (by T cells)

2. Antibody mediated or humoral immunity (by B cells)

The large and complex foreign molecules (mostly proteins) that activate the specific immunity are known as antigens. Body immune systems recognize a large variety of antigens readily. However, antigenic determinants are those sites on antigens that are recognized by antibodies and receptors present on T cells and B cells.

Cell-mediated immunity:

In this type of immunity, the subgroup of T cells called T cytotoxic cells, which are specific to a target cell (infected cell), are activated. The T cytotoxic cells destroy the target cells and therefore, prevent the completion of the life cycle of the pathogen. This kind of immunity also kills cancer cells.

Antibody-mediated immunity:

The B cells produce specialized proteins called antibodies, which are highly specific to a particular antigen. Antibodies are collectively termed as immunoglobulins or Ig. Each immunoglobulin molecule is made up of 4 polypeptide chains: the two long chains are called Heavy or H chains and two short chains are called light or L chains. These four polypeptide chains are held together to form a Y shaped structure. The top two tips of this Y shaped molecule bind to the specific antigens in a lock and key fashion, thus forming an antibody-antigen complex. The B cells thus direct the antibody-mediated immunity, which is also called humoral immunity.

Reference Section

Microbiologybytes. “Innate and Acquired Immunity.” Retrieved on December 4, 2008 from Acquired immunity.