U4 AOS1 Topic 3: Third Line of Defense

The third line of defense is a component of adaptive immunity which has a specific immune response and form immunological memory.

Initiation of adaptive immunity:

·        The third line of defense comes into action when antigen presenting cells (APCs) engulf the pathogens and display the pathogenic antigens on their specific markers called MHC class II markers.

·        APCs then travel via the lymphatic system into the lymph nodes where helper T cells are present.

·        Antigens on APCs interact with T cell receptors on helper T cells to make them active.

The activated helper T cells then initiate the adaptive immune response via humoral or cell mediated immune response.

1.  Humoral immunity:

Humoral immunity involves the destruction of pathogens by producing antibodies against specific pathogenic antigens. A type of white blood cells known as B lymphocytes has B cell receptors on their surface known as antibodies.

·        B cell lymphocytes become activated when their antibodies interact with antigens. Antigens bind to their complementary site known as antigen-binding site on an antibody and the respective B cell is said to be “selected”. The selection of B cell together with helper T cell is termed as “clonal selection theory”.

·        Both B cell and helper T cell have complementary receptor to the antigen. The selected helper T cell interacts with selected B cell and secretes cytokines to stimulate “clonal expansion” where multiple copies of B cells are produced. Cytokines also stimulates the differentiation of B cell into B memory cells and effector cells.

·        After differentiation, the effector cells also known as plasma cells release antibodies into the bloodstream to defend against a specific antigen whereas, the memory B cells participate in immunological memory and remain in blood for an extended period until the specific antigen is introduced in the body again.

2.  Cell-mediated immunity:

·        During the selection of T helper cell there is another type of cell known as naive T cell which interacts with antigen presenting cell and becomes “selected”. Cytokines released from helper T cell stimulate clonal expansion and differentiation.

·        In clonal expansion, multiple copies of T cells are produced and differentiated into two types of cells i.e. effector cell and memory T cell.

·        The effector cell also known as cytotoxic T cell reaches the site of infection and interacts with abnormal or infected cell that is presenting the specific antigen complementary to the T cell receptor. Upon binding of antigen to T cell receptor of cytotoxic T cell, a chemical called perforin is released by cytotoxic T cell which causes apoptosis or programmed cell death.

·        Memory T cells remain in blood for a long period of time and help in creating immunological memory to defend against the pathogen when it re-enters the body in future.