Stages/Phases of infections

The innate response occurs within 0-4 hours, where preformed mediators are released, this recruits and activates the effector cells (i.e.: NK, phagocytes, complement, acute phase proteins etc) – as a result – these results migrate to the site of infection in an activated form and remove the infectious agent. After this (4-96 hrs) – if the infection is still present, then more recruitment of inflammatory cells occurs and more chemical mediators are secreted. These cells recognise the infectious agent and get activated – and as a result increased inflammation results and the infectious agent is removed. After 96 hours, the antigen may make its way (i.e.: APCs present it to them) to the lymph nodes. It is then recognised by naïve T cells, which are activated after the following occurs: T cells recognise ligands on APCs, TCR recognises MHC-associated peptide antigens, CD4/CD8 co receptors recognise MHC molecules, adhesion molecules strengthen binding of T cells to APCs, costimulation provided by APCs. This cause’s clonal expansion, where T lymphocytes begin to proliferate + differentiate, resulting in expansion of antigen specific clones. CD4+ helper T cells may differentiate into effector cells that secrete cytokines mainly to activate macrophages and B lymphocytes & CD8+ T cells may differentiate into CTLs that kill infected cells expressing the antigen. This results in removal of infectious agent. But memory is preserved after this. So in the event of a re-infection, preformed antibody and memory T cells recognise the antigen – rapid expansion and differentiation into effector cells occurs – together they remove the infectious agent.

Category: Pathology Notes