Bacteria enters the host either through natural openings/routes such as respiratory tract, gastrointestinal tract , urinary tract etc. or through damaged natural barriers such as skin and mucosa layer.
The immune response to bacteria depends on the size, number, virulence etc. of the pathogen entered in to the body. The common response to extra cellular bacteria is antibody mediated (humoral response), but delayed type hypersensitivity is employed in case of intracellular bacteria.
Pathogenic mechanisms of bacteria may be direct through cell rupture or by causing inflammation, or by means of exotoxins (soluble molecules released by bacteria to trigger pathological change in host) or by end toxins such as lip polysaccharides that can trigger inflammation.
Immune responses to pathogens can be carried by inflammation for lysis of infected cells; killing of uninfected cells also takes place resulting in tissue damage and resulting disease. Rheumatic fever is the best example for this kind of disease. Innate immunity prevails initially, with the activation of macrophages and inflammatory cells.
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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 also activated to eradicate the pathogens. This causes an immediate inflammatory response that keeps the bacteria in check before other processes take over. In the later period the adaptive immune response takes over, and involves specific antibody production.
Antibodies prevent infection by blocking the ability of attachment of microbes to the host cells for invasion. In addition to that antibodies neutralize infectivity and pathogen city of released toxins by binding to them and interfering with their functions.
Antibodies opsonize microbes to promote their phagocytosis and activate the classical complement pathway.
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Certain bacteria such as Clostridia or B. anthracis releases toxic products in to the host body and the toxic products are responsible for pathogenesis. Neutralization of toxic products such as Clostridium botulinum type B, anthrax toxin etc. is essential to save the body and regulate normal metabolic activities.
Hence the immune system neutralizes the toxic products in addition to killing/ removing parasites with the help of antibodies. Neutralization occurs as a result of blocking of the union of the toxin and its receptor on the host cell. IgG antibodies are totally responsible for toxin neutralization.
Cell mediated immune response takes control of intracellular 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 + cells, cytokines etc. CDg + T cells on the other hand differentiate into cytotoxic cells and directly kill the infected cell (bacteria within cytoplasm of cell) thus eliminating the infection.