Immunology | immunity | Innate (non-specific) and Adaptive (specific) immunity

Introduction

Immunity or resistance is the ability to ward off damage or disease through our defenses

The two general types of immunity

ü  Innate (nonspecific) immunity refers to defenses that are present at birth.  Among the components of innate immunity are the first line of defense (the physical and chemical barriers of the skin and mucous membranes) and the second line of defense (antimicrobial substances, natural killer cells, phagocytes, inflammation, and fever).

ü  Adaptive (specific) immunity refers to defenses that involve specific recognition of a microbe once it has breached the innate immunity defenses. Adaptive immunity is based on a specific response to a specific microbe; that is, it adapts or adjusts to handle a specific microbe. Adaptive immunity involves lymphocytes (a type of white blood cell) called T lymphocytes (T cells) and B lymphocytes (B cells).

INNATE IMMUNITY

First Line of Defense :

            The skin and mucous membranes of the body are the first line of defense against pathogens. These structures provide both physical and chemical barriers that discourage pathogens and foreign substances from penetrating the body and causing disease.

Second Line of Defense:

            Antimicrobial Substances :

ü  Interferons,

ü  complement,

ü  Iron-binding proteins, and

ü  antimicrobial proteins

·         dermicidin - produced by sweat gland,

·         defensins and cathelicidins - produced by neutrophils, macrophages, and epithelia

·         thrombocidin - produced by platelets

            Natural Killer Cells and Phagocytes

ü  NK cell Kill infected target cells by releasing granules that contain perforin and granzymes; phagocytes then kill released microbes.

ü  Phagocytosis occurs in five phases : chemotaxis, adherence, ingestion, digestion, and killing.

ADAPTIVE IMMUNITY:

            The ability of the body to defend itself against specific invading agents such as bacteria, toxins, viruses, and foreign tissues is called adaptive (specific) immunity.

            Adaptive immunity involves lymphocytes called B cells and T cells.

Cell-mediated and antibody-mediated immune response

CELL-MEDIATED IMMUNITY:

Activation of T Cells

Antigen receptors on the surface of T cells, called T-cell receptors (TCRs), à recognize and bind to specific foreign antigen fragments that are presented in antigen–MHC complexes. When an antigen enters the body, only a few T cells have TCRs that can recognize and bind to the antigen. à  Antigen recognition also involves other surface proteins on T cells, the CD4 or CD8 proteins. à These proteins interact with the MHC antigens and help maintain the TCR–MHC coupling. à A T cell becomes activated only if it binds to the foreign antigen and at the same time receives a second signal, a process known as costimulation.

Activation and Clonal Selection of Helper T Cells

The helper T cell undergoes clonal selection à The result is the formation of a clone of helper  T cells that consists of active helper T cells and memory helper T cells.  à Within hours after costimulation, active helper T cells start secreting a variety of cytokines à One very important cytokine produced by helper T cells is interleukin-2 (IL-2)

Activation and Clonal Selection of Cytotoxic T Cells:

Maximal activation of cytotoxic T cells requires presentation of antigen associated with both MHC-I and MHC-II molecules. à Active cytotoxic T cells attack other body cells that have been infected with the antigen.

Elimination of Invaders:

Cytotoxic T cells recognize and attach to target cells à Cytotoxic T cells are the soldiers that march forth to do battle with foreign invaders in cell-mediated immune responses.

ANTIBODY-MEDIATED IMMUNITY:

Activation and Clonal Selection of B Cells

            During activation of a B cell, an antigen binds to B-cell receptors à The antigen is taken into the B cell, broken down into peptide fragments and combined with MHC-II self-antigens, and moved to the B cell plasma membrane. à Helper T cells recognize the antigen–MHC-II complex and deliver the costimulation needed for B cell proliferation and differentiation. à The helper T cell produces interleukin-2 and other cytokines that function as costimulators to activate B cells à Different antigens stimulate different B cells to develop into plasma cells and their accompanying memory B cells. All of the B cells of a particular clone are capable of secreting only one type of antibody, which is identical to the antigen receptor displayed by the B cell that first responded à Each specific antigen activates only those B cells that secrete antibody specific to that antigen. Antibodies produced by a clone of plasma cells enter the circulation and form antigen–antibody complexes with the antigen that initiated their production.