Third Line of Defence The third line of defence against pathogenic invasion is the adaptive immune response, which has two key qualities: It is specific (it can differentiate between specific microorganisms and respond accordingly) It is adaptive (it can produce a heightened response upon re-exposure - in other words, it has memory) Antigen Presentation An antigen is a substance that the body recognises as foreign and can evoke an immune response The immune system can differentiate between foreign ('non-self') and native ('self') cells due to the presence of identification markers (MHC molecules) All body cells (excluding red blood cells) possess MHC class I markers on their cell surface, identifying them as part of the organism Red blood cells do not have noticeable MHC 1 molecules. But rather the have high levels of CD47 markers which are recognized by the immune system as self. On the other hand red blood cells during development are nucleated they may contain small traces of MHC 1 molecules. Certain immune cells (e.g. macrophages) possess MHC class II markers, which present foreign substances to the adaptive immune system The body is initially capable of recognising invaders as they do not possess the molecular markers that designated them as 'self' (MHC class I) When non-specific macrophages engulf a pathogen and destroy it (via the lysosome), they present antigenic fragments on their surface, complexed with MHC class II molecules ('non-self') This allows cells of the adaptive immune system (i.e. lymphocytes) to generate a specific response against that particular antigenic determinant Infected body cells (e.g. viruses and tumors) may present antigenic fragments on MHC class I molecules, denoting that the cell is now compromised and needs to be destroyed
CELL MEDIATED IMMUNITY
Cell mediated immunity involves the action of T cells. T cells are a group of lymphocyte which originate in the bone marrow but mature and differentiate in the thymus gland, attacking pathogens that have invaded the cells of the body. T cells contain T cell receptors which bind to antigens (specific). The four types of T cells that exist are T cells involved are helper T cells (TH), cytotoxic T cells (TC), suppressor T cells (TS) and memory T cells (TM¬). T cells have receptors which foreign antigen matter bind to and initiate further response from other cells. Helper T Lymphocytes (TH cells) When a pathogen invades the body, it is engulfed by wandering macrophages which present the antigenic fragments on its surface This macrophage becomes an antigen-presenting cell, and presents the antigen to helper T cells (TH cells) The TH cells bind to the antigen and become activated, and in turn activate the B cell with the specific antibody for the antigen This B cell clones and differentiates into plasma cells and memory cells Antibody Production via the Activation of Helper T cells Cytotoxic T Lymphocytes (TC cells) Cytotoxic T cells recognise antigenic fragments on infected cells (bound to MHC-I markers) and kill these cells before the virus has time to replicate Some TC cells can even destroy certain types of cancer cells Cytotoxic T cells destroy infected cells by releasing lymphotoxins, which cause cell lysis Once the infected cells have been destroyed, suppressor T cells inhibit the TC cells, to ensure that normal cells are not subsequently attacked TC cells (adaptive immunity) differ from natural killer cells (innate immunity) in their ability to form memory cells for subsequent reinfections Suppressor T cells (T¬S) Suppressor T cells regulate immune response and stops the activity of the immune system once the invader has been defeated to save resources by the secretion of proteins and enzymes Cytotoxic T cells (TC) Cytotoxic T cells recognise and destroy infected, cancer and foreign eukaryotic cells, releasing perforin to lyse the cells and toxins to destroy the cell and its contents. The releases of other cytokines further stimulates macrophages to undergo phagocytosis. TC cells also release interferon. Memory T cells (TM) Have the ability to quickly recognise foreign antigens so that a rapid response occurs with subsequent invasions #AcquiredImmunity #ThirdLineOfdefense