When a pathogen penetrates the chemical and physical barriers of the first line of defence and enters the
internal environment, the defence mechanisms are activated. The innate immune system is the first to respond.
Although slower to start, the acquired immune system activates mechanisms aimed specifically at the pathogen.
In this module you will follow the basic pattern of response to a bacterial infection, a viral infection, an allergen,
and an incompatible blood type.
In Unit 1 you were introduced to the tissues, cells and mechanisms of the immune system. In Unit 2, the
basic response pathways to different types of pathogens will be examined.
The immune response to bacteria is illustrated in figure 2417. The final goal of this pathway is
destruction of the bacteria. To this end, complement proteins, acute phase proteins and antibodies for the
particular bacteria are made available to tag the bacteria and aid in its ingestion by phagocytes. The figure
shows how the different components of the immune system function together to produce the appropriate defence.
Note that communication is necessary to coordinate the interconnected pathway.
RESPONSE TO BACTERIAL INFECTION:
• If passive barricades of skin & mucous membranes fail, bacteria reaches the ECF, causing inflammation
• Bacteria usually elicit a nonspecific inflammatory response
o In addition, lymphocytes produce antibodies keyed to specific type of bacterium
1. Activity of the complement system
i. Complement proteins act as chemotaxins that attract leukocytes to help fight infection
ii. Others act as opsonins to enhance phagocytosis
iii.Causes degranulation of mast cells and basophils
iv. Histamine increases capillary permeability
v. Plasma proteins lead to tissue edema
vi. Ends with formation of membrane attack complex molecules
2. Activity of phagocytes
i. Unencapsulated bacteria are ingested by macrophages
i. Opsonins enhance process
ii. Encapsulated bacteria are coated in antibodies before being ingested by phagocytes
3. Role of the acquired immune response
i. If antibodies already present, innate responses enhanced by acting as opsonins and neutralizing
ii. APC’s that digest bacteria that move to 2 lymphoid tissues
i. Present antigen to memory cells to initiate more antibody production
4. Initiation of repair
i. If initial wound damaged blood vessels under skin, platelets and proteins of coagulation are
recruited to minimize the damage
ii. Once bacteria removed, injured site is repaired by growth factors and other cytokines
1 Module XV
The pattern of immune response to a viral infection is depicted in figure 2418. Viruses infect cells. The
innate natural killer cells and specific cytotoxic T cells attack and destroy infected cells, preventing the spread of
the infection. Viruses in the extracellular fluid may be ingested by macrophages and initiate the pathway for
creation of antibodies against the virus. In some cases antibody coating of the virus will prevent the virus from
entering uninfected cells.
RESPONSE TO VIRAL INFECTION:
• Innate immune responses and antibodies help control the early stages of a viral infection
• Once the viruses enter the host’s cells, cytotoxic T and NK cells must destroy infected host cells
1. Antibodies act as opsonins
a. However, once virus is inside host cells, antibodies are no longer effective
2. Macrophages that ingest viruses insert fragments of viral antigen on their membranes
a. Secrete cytokines to initiate the inflammatory response
b. Make antiviral proteins to