29.1 General features of immunity to microbes

•  Microbial infections are best prevented by both innate and adaptive immune responses. The innate immune system takes care of early defense while the adaptive immune system offers a longer and potential response. In addition, adaptive immune responses are more specific and confer protection from repeated attacks by producing memory cells.

•  As microbes differ a lot in their host attacking regime, their removal from the affected patient requires efficient effector systems. Adaptive immunity is developed in such a way that it permits the affected host to respond favorably to different types of microbes.

•  The result of many microbial infections is decided by the balance between microbial schemes for withstanding immunity and the host immune responses. Microbes have adapted several ways to combat the immune response.

•  Immunity against microbes performs almost similar to other defense mechanisms. Although it is essential for host survival but sometimes it may cause damage to the host tissue itself.

•  Some microbes especially viruses have the potential to be latent. In such cases the host immune response does not allow the microbe to spread but the microbes survive in the latent form, i.e. infection may prevail under specific conditions like stress etc.

29.2 Immunity to bacteria

29.2.1 Immunity to extracellular bacteria

Extracellular bacteria are those that multiply and reside outside the host cell. These bacteria mainly affect the cells in two ways. They either attack by causing inflammation and tissue damage or by producing toxins.

29.2.2 Innate immunity to extracellular bacteria

Innate immunity to extracellular bacteria essentially involves three processes.

•  Stimulation of phagocytes - Phagocytes takes the help of surface receptors and Fc receptors to identify extracellular bacteria and its opsonization with the help of antibodies, respectively. Most of these receptors are associated with promotion of phagocytic activity and microbicidal activity.

•  Induction of inflammatory response - Antigen presenting cells like dendritic cells in addition to phagocytes are stimulated by microbes and these cells secrete cytokines which are responsible for causing leukocyte infiltration at the site of inflammation.

•  Activation of complement system - Both gram positive and gram negative bacteria stimulate alternative pathway of complement system and mannose expressing bacteria stimulate lectin pathway of complement system by binding to mannose binding lectin.