Page 142 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

Immunoglobulins

Definition

Glycoprotein molecules are produced by plasma cells in response to an

immunogen (antigen).

Function

Antibodies are the primary defense against extracellular

pathogens, and they function in three major ways:

1- Complement activation
• The binding of antibody to antigen can trigger the classical pathway

of the complement cascade.

2- Neutralization

A toxin or virus coated with antibodies is prevented from

interacting with a cell and therefore can no longer cause damage.

3- Opsonization

• Macrophages and neutrophils both have receptors for the Fc region

of these antibodies on their surface, facilitating the attachment of the

phagocytic cell to the antibody-coated antigen as a prelude to

engulfment.

Notes

• Immunoglobulins bind specifically to one or a few closely related antigens. Each

immunoglobulin actually binds to a specific epitope or antigenic determinant (that

portion of an antigen that combines with the immunoglobulin).

• Valency - The valency of antibody refers to the number of antigenic determinants that
an individual antibody molecule can bind. The valency of all antibodies is at least two

and in some instances more.

General Microbiology & Immunology (PM 401) theoretical handbook Page 131