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).
by Gary E. Kaiser, Ph.D.
Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work at http://faculty.ccbcmd.edu/~gkaiser/index.html.Last updated: August, 2019
Please send comments and inquiries to Dr. Gary Kaiser
b. With enveloped viruses, the MAC can damage the viral envelope (see Fig. 3 and Fig. 4).
by Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.Last updated: Feb., 2020
Please send comments and inquiries to Dr. Gary Kaiser
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).
by Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.Last updated: Feb., 2020
Please send comments and inquiries to Dr. Gary Kaiser
However, as learned in Unit 3, some bacteria by means of the activities described below are more resistant to MAC lysis.
- The LPS of the cell wall is the principle target for complement in Gram-negative bacteria by activating the alternative complement pathway (def) and serving as a binding site for C3b as well as the site for formation of MAC. Some Gram-negative bacteria attach sialic acid to the LPS O antigen (see Fig. 7) and this prevents the formation of the complement enzyme C3 convertase that is needed for the eventual formation of all the beneficial complement proteins such as C3b, C5a, and MAC. Blood-invasive strains of Neisseria gonorrhoeae (inf), as well as Bordetella pertussis (inf) and Hemophilus influenzae (inf) are examples of Gram-negative bacteria that are able to alter their LPS in this manner.
- Some Gram-negative bacteria, such as Salmonella (inf), lengthen the LPS O antigen side chain (see Fig. 8) and this prevents the formation of MAC. Neisseria meningitidis (inf) and Group B Streptococcus (inf), on the other hand, produces capsular polysaccharides composed of sialic acid and as mentioned above, sialic acid prevents MAC lysis.
- An outer membrane molecule of Neisseria gonorrhoeae (inf) called Protein II binds factor H of the complement pathway and this leads to the degradation of the opsonin C3b by factor I and the formation of C3 convertase. Without C3 convertase, no MAC is produced.