Question

How does the body manufacture antibodies for a specific pathogen? How do they recognize which ones will work against a particular one if they've never previously encountered it?

Answer 1

Antibody-mediated immune response:

Antibody-mediated immune response (AMI) provides protection to the host by secreting antibodies; that prevent invasion of microbes present on the surface of the host cells and in the extracellular environment, but has no role against intracellular microbes. AMI occurs through the following three sequential steps:

Recognition:

Activation of B cells following contact with the microbial antigen (B cells act as APC). Proliferation and differentiation of B cells into effector cells (antibody-producing plasma cells) and memory cells.

Effector function:

Production of secreted antibodies by plasma cells which in turn counteract with the microbes in many ways, such as neutralization, opsonization, complement activation, etc.

Activation of B Cells:

Most antigens are thymus-dependent; they activate B cells indirectly via activation of T cells. TD antigens are processed by APCs, presented T-helper cells following which the activated T-h cells secrete cytokines that in turn activate the B cells.

Thymus independent antigens are not processed by APC. They carry directly activate B cells without the help of T cell-induced cytokines.

Steps involved in recognition of microbial antigen:

Antigen Presentation of B Cells to Activated T-h cells. The first and foremost step is the recognition of microbial antigen by B cell membrane. The antigen is processed into smaller antigenic peptides that are presented in complex with MHC-11 to activated T-h cells.

Signal Induction: The naive B cells get activated.

Proliferation and differentiation of B cell:

The naive B cells, released from bone marrow go and house in the B cell areas of peripheral lymphoid organs ( Cortex of lymph node and marginal zone of the spleen). Following the antigenic exposure, the naive B cells are activated and then they proliferate. Eventually, the primary lymphoid follicles transform into secondary lymphoid follicles. Secondary lymphoid follicles bear a germinal center that has a dark zone and light zone.

Events in the Dark Zone of Germinal Center:

Activated B cells differentiate into larger dividing cells called centroblasts, which further transform into smaller non-dividing cells called centrocytes by expressing membrane lg.

Events in the light zone of germinal center:

Class switch over: Early in the immune response, IgM is the predominant immunoglobulin secreted by the B cells. But as the maturation progresses, the same B cells undergo a phenomenon called class switch over.

After undergoing class switch over, the selected centrocytes further undergo differentiation into effector cells (plasma cells) and memory cells in the light zone of the germinal center.

Plasma cells produce large antibody-secreting cells. They do not have MHC-2 molecules and do not undergo the class switch over.

Memory cells affinity membrane immunoglobulin molecules of all classes as compared to naive B cells. They are long-lived which respond to secondary antigenic stimulus.

Effector functions of antibody-mediated immunity:

Opsonization, transcytotic activities, complement-mediated inflammation, and cytolysis mucosal immunity.