THE ADAPTIVE IMMUNE SYSTEM

II. HUMORAL IMMUNITY

B. WAYS THAT ANTIBODIES HELP TO DEFEND THE BODY

3. MAC Cytolysis

Fundamental Statements for this Learning Object:

1. The Fab portion of IgG or IgM reacts with the epitopes on the membrane and the Fc portion of the antibody then activates the classical complement pathway. C5b6789n (the membrane attack complex or MAC) then puts holes in the membrane.
2. In the case of bacteria, MAC can put holes in the outer membrane and possibly the cytoplasmic membrane of the Gram-negative cell wall causing lysis.
3. In the case of enveloped viruses, MAC can damage the viral envelope.
4. In the case of human cells recognized as nonself - virus-infected cells, transplanted cells, transfused cells, cancer cells- the MAC causes direct cell lysis.

 

LEARNING OBJECTIVES FOR THIS SECTION


Humoral Immunity refers to the production of antibody molecules in response to an antigen (def). These antibody molecules circulate in the plasma of the blood and enter tissue and organs via the inflammatory response. Humoral immunity is most effective microbes or their toxins located in the extracellular spaces of the body.

Antibodies or immunoglobulins (def) are specific glycoprotein configurations produced by B-lymphocytes and plasma cells in response to a specific antigen and capable of reacting with that antigen.

The antibodies produced during humoral immunity ultimately defend the body through a variety of different means. These include:

1. Opsonization
2. MAC Cytolysis
3. Antibody-dependent Cellular Cytotoxicity (ADCC) by NK Cells
4. Neutralization of Exotoxins
5. Neutralization of Viruses
6. Preventing Bacterial Adherence to Host Cells
7. Agglutination of Microorganisms
8. Immobilization of Bacteria and Protozoans
9. Promoting an Inflammatory Response


In this section we will look at MAC cytolysis.


3. MAC Cytolysis (def)

The process starts with the antibody isotypes IgG or IgM being made against epitopes on membranes. The Fab portion (def) of IgG or IgM reacts with the epitopes on the membrane and the Fc portion (def) of the antibody then activates the classical complement pathway. C5b6789n (the membrane attack complex or MAC) (def) then puts holes in the membrane. (Remember that MAC is also produced during the alternative complement pathway and the lectin pathway as was discussed in Unit 5.)

a. In the case of bacteria, MAC can put holes in the outer membrane and possibly the cytoplasmic membrane of the Gram-negative cell wall (see Fig. 1) causing lysis (see Fig. 2).

 

b. With enveloped viruses, the MAC can damage the viral envelope (see Fig. 3 and Fig. 4).

 

c. In the case of human cells recognized as nonself- virus-infected cells, transplanted cells, transfused cells, cancer cells - the MAC causes direct cell lysis (see Fig. 5 and Fig. 6).

     

 

 

However, as learned in Unit 3, some bacteria by means of the activities described below are more resistant to MAC lysis.

 

 


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