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