Microbial immunity

Microbial immunity
The immune system has developed to protect the host from pathogens and other foreign substances, self/non-self discrimination is one of the hallmarks of the immune system. Most pathogens can elicit both an antibody (humoral) and a cell-mediated response, both of which may contribute to ridding the host of the pathogen. However, for any particular pathogen one type of immune response whether antibody or a cell-mediated immune response may be more important than the other type for the host to defend against the pathogen.
Defense against infections is mediated by the early reactions of innate immunity and the later responses of adaptive immunity. The immune system is specialized to generate different effector mechanisms for different types of microbes. The survival and pathogenicity of pathogens in a host and so the outcome of infections is determined by the balance between host defenses and the ability of microbes to evade or resist immunity. Immune responses to microbes are themselves capable of causing tissue injury (Immunopathology). Antigenic mimicry is where a protein sequence of a foreign antigen is similar to a self-protein which results in unintentional cross reactivity of antibodies or T cell epitopes. This pathology can occur long after infectious agent has been eliminated (e.g. rheumatic fever after strept. infection).

Immune responses in viral infections
Viruses are obligatory intercellular pathogens that replicate within cells. They use the nucleic acid and protein synthetic machineries of the host cell. It can infect a variety of cell populations by utilizing normal cell surface molecules as receptors to enter cell. Infected cells use their major histocompatibility complex molecules to put pieces of the invading microbes on their surfaces, flagging down cytotoxic T lymphocytes to destroy the infected cells. ?The primary defense against viruses is cell-mediated responses CMI, the cytotoxic T lymphocyte. Antibodies also can assist in the immune response by opsonizing and blocking viruses before they have a chance to enter cells.
Innate immunity:
1. The principal mechanisms of innate immunity against viruses are inhibition of infection by interferons (INF). Interferons are antiviral proteins, or glycoproteins produced by several types of cells in response to viral infection: INF? by leucocytes, INF? by fibroblast, and INF? by natural killers (NK) cells. IFNs function to inhibit viral replication in both infected and uninfected cells.
2. NK cell –mediated killing of infected cells. NK cells are the host adaptation for killing class I MHC-negative virus infected cells.