Immunity to bacteria

Bacteria can be gram +ve or gram –ve. Gram +ve bacteria are opsonised and phagocytosed. Gram –ve bacteria are opsonised and undergo lysis by the complement mechanism. Intracellular bacteria are killed by macrophage activation by T cell derived cytokines, and also CTLs directly killing the infected cell. If the bacteria is non-invasive, then a humoral response (antibody production) is enough to neutralise the toxin.

Antibody

Humoral immunity is mediated by antibodies. Antibodies prevent infection by blocking the ability of microbes to invade host cells. These can be done by interfering with the microbe’s ability to attach to the host tissues. Antibodies have the following functions: neutralise infectivity and pathogenicity of toxins by binding to them and interfering with their functions, opsonise microbes therefore promoting phagocytosis, activate the classical complement pathway (stimulate inflammation / MAC). The alternate pathway does not require antibody to be activated (immediate response).

Extracellular bacteria

Extracellular bacteria are first (0-4hrs) handled by the innate immune response. Macrophages and other cells secrete cytokines and chemokines that attract inflammatory cells to the site of infection. Alternate complement pathway is activated. This causes an immediate inflammatory response which keeps the bacteria in check before other processes take over. The adaptive immune response takes over, and this involves antibody production due to B cell activation  plasma cells, and cytokines released due to CD4+ T cells activated + CTLS from CD8+ T cells being activated. The antibodies produce further induce classical complement pathway activation. Therefore together they attack the bacteria.

Intracellular bacteria

Cell mediated immune response take control of this type of bacteria. The bacteria can be located within vesicles in cells, or within the cytoplasm itself. Intracellular bacteria residing in vesicles within phagocytes are removed by activated phagocytes. This activation is provided by CD4+ T cells, production of cytokines etc. CD8+ T cells on the other hand differentiate into CTLs and directly kill the infected cell (bacteria within cytoplasm of cell) thus eliminating the infection.

The innate immunity prevails again initially, with the activation of macrophages and inflammatory cells.

Category: Pathology Notes