The Immune System

Immune System Our immune system is made up of: The innate immune system: first line of defence (non-specific) The adaptive immune system: more sophisticates defence system (specific to the pathogen it targets).

Innate Immunity (Non-specific) The innate immune system responses are general defence systems of the body. i.e. They are not specific This is the first line of defence and the second line of defence

Innate Immunity: The Skin The skin is the primary defence of the body. The protective boundaries of some other human tissues (eyes, nasal passages, respiratory tract, mouth, digestive tract, urinary tract, and female genital tract) are less robust. In the lungs and small intestine, the barrier is just a single monolayer of epithelial cells. Pathogens must cross this protective barrier in order to colonise a host. Other first line defence mechanisms include mucous membranes, natural secretions and natural flora (of the body).

Second Line of Defence Once the body’s first line of defence has been compromised, the second line of defence is employed. This may include: Phagocytes and killer cells Complement proteins Interferon Cytokines Inflammation

Adaptive Immune System (Specific ) The adaptive immune system responses are highly specific to a particular pathogen, and they provide long-lasting protection against them. Adaptive immune responses eliminate or destroy invading pathogens and any toxic molecules they produce. As these responses are destructive, it is important that they are directed only against foreign molecules and not against molecules of the host itself. Many harmless foreign molecules enter the body, and it would be pointless and potentially dangerous to mount adaptive immune responses against them.

Adaptive Immune System The adaptive immune system uses multiple mechanisms to avoid damaging responses against self molecules. Occasionally these mechanisms fail, and the system turns against the host, causing autoimmune diseases, which can be fatal.

Allergies Allergic conditions such as hayfever and allergic asthma are examples of adaptive immune responses against apparently harmless foreign molecules.

Antigens Any substance capable of eliciting an adaptive immune response is referred to as an antigen. The adaptive immune system can distinguish between antigens that are very similar—such as between two proteins that differ in only a single amino acid.

The Innate and Adaptive Immune Systems Work Together Lymphocytes usually respond to foreign antigens only if the innate immune system is first activated. the rapid innate immune responses to an infection depend largely on pattern recognition receptors made by cells of the innate immune system.

Lymphocytes Lymphocytes are responsible for the specificity of adaptive immune responses. They occur in large numbers in the blood and lymph (the colorless fluid in the lymphatic vessels that connect the lymph nodes in the body to each other and to the bloodstream). They are also concentrated in lymphoid organs, such as the thymus, lymph nodes (also called lymph glands), spleen, and appendix.

Lymphocytes

Adaptive immune responses are carried out by white blood cells called lymphocytes. There are two broad classes of such responses antibody responses T-cell-mediated immune responses There are also different classes of lymphocytes, called B cells and T cells which carry out the above responses, respectively.

Antibody Responses In antibody responses, B cells are activated to secrete antibodies, which are proteins called immunoglobulins. The antibodies circulate in the bloodstream and permeate the other body fluids, where they bind specifically to the foreign antigen that stimulated their production. Binding of antibody inactivates viruses and microbial toxins (such as tetanus toxin or diphtheria toxin) by blocking their ability to bind to receptors on host cells. Antibody binding also marks invading pathogens for destruction, mainly by making it easier for phagocytic cells of the innate immune system to ingest them.

Antibodies (Immunoglobulins)

Immunoglobulins There are different classes of immunoglobulins IgM antibodies – first to be secreted in an infection. Cause agglutination of cells containing antigens. IgG antibodies – activate complement proteins in the blood & neutralizetoxins. (Most common antibody that passes between mother and baby.) IgA antibodies – neutralise pathogens in respiratory, digestive and reproductive tracts. IgE antibodies – help initiate inflammation after pathogens infect. Also causes allergic reactions to non-pathogenic agents.

T-Cell Mediated Immune Responses In T-cell-mediated immune responses, activated T cells react directly against a foreign antigen that is presented to them on the surface of a host cell. T cells can detect microbes hiding inside host cells and either kill the infected cells or help the infected cells or other cells to eliminate the microbes. The T cell, for example, might kill a virus infected host cell that has viral antigens on its surface, thereby eliminating the infected cell before the virus has had a chance to replicate. In other cases, the T cell produces signal molecules that either activate macrophages to destroy the microbes that they have phagocytosed or help activate B cells to make antibodies against the microbes.

Summary A summary of the two types of adaptive immune responses Antibody Responses T-Cell Mediated Responses

Recognising Self and Non-self B Cells and T Cells need to be able to distinguish between ‘self’ and ‘non-self’. ‘Self’ cells are determined by proteins on the cell membrane. Proteins on cell membranes are determined by genes These genes code for major histocompatibility complex (MHC) and these proteins are called markers. All cells have MHC on their surfaces. B and T cells recognise and ignore cells that have the same MHC markers as themselves.

MHC Markers There are 2 types of MHC markers. Class 1 MHC markers are found on the surface of all nucleated cells (all body cells except red blood cells) Class 2 MHC markers are found on the surface of B cells, T cells and macrophages. MHC markers also have a role in governing the success of organ transplants (the immune system is suppressed to prevent rejection of foreign tissue).