HUMORAL IMMUNE RESPONSE
DEFINITION
Humoral immunity or humoural immunity is the aspect of immunity that is mediated by macromolecules found in extracellular fluids such as secreted antibodies, complement proteins, and certain antimicrobial peptides.
MECHANISM
In humoral immune response, first the B cells mature in the bone marrow and gain B-cell receptors (BCR's) which are displayed in large number on the cell surface.
These membrane-bound protein complexes have antibodies which are specific for antigen detection. Each B cell has a unique antibody that binds with an antigen. The mature B cells migrate from the bone marrow to the lymph nodes or other lymphatic organs, where they begin to encounter pathogens.
B cell activation
When a B cell encounters an antigen, the antigen is bound to the receptor and taken inside the B cell by endocytosis. The antigen is processed and presented on the B cell's surface again by MHC-II proteins.
B cell proliferation
The B cell waits for a helper T cell (TH) to bind to the complex. This binding will activate the TH cell, which then releases cytokines that induce B cells to divide rapidly, making thousands of identical clones of the B cell.
Antibody-antigen reaction
these antibodies will encounter antigens and bind with them. This will either interfere with the chemical interaction between host and foreign cells, or they may form bridges between their antigenic sites hindering their proper functioning, or their presence will attract macrophages or killer cells to attack and phagocytose them.
ROLE OF COMPLEMENT SYSTEM
The complement system is the ability of antibodies and phagocytic cells to remove pathogens from an organism.
Classical Complement Pathway
The antibody binds to an antigen on the surface of a pathogen, activating the C1 complement protein.
C1 acts a protease and cleaves C2 and C4 to form C4b2b.
C42b converts C3 into C3a and C3b, which forms a C5 convertase.
C5 convertase cleaves C5 into C5a and C5b.
C5b forms a complex with C6, C7, and then C8, and C9, which becomes the membrane attack complex that lyses the pathogen.
The Alternative Complement Pathway
The pathogenic antigen (such as LPS) activates C3 so it creates a C3B complex
Factor D cleaves the C3B complex so that C3bBb is created.
C3bBb is a C3 convertase, which converts more C3 into C3a and C3b.
Similarly to the classical pathway, C3b forms a C42b complex, and the rest of the steps are essentially the same as the classical pathway, ending with C5b forming a membrane attack complex with C6, C7, C8, and C9.
Lectin Pathway
MBL binds to the carbohydrates on a pathogen.
Proteases bound on the other side of the MBL cleaves C4 into C4a and C4b.
C4b creates C3 convertase, and the rest of the steps happen identically to the classical pathway from the C3 convertase step.
IMMUNOLOGICAL MEMORY
Immunological memory refers to the ability of B and T cells to produce long-lived memory cells that defend against specific pathogens.
Passive Memory
Newborn infants are particularly vulnerable to infections since they have no prior exposure to pathogens.
Since the fetus isn’t making any memory cells or antibodies, this is called passive immunity. Passive immunity is short-lived, ranging from a couple days to a couple months.
Active Memory and Immunization
Following an infection, long-term active memory is acquired by activation of B and T cells
Memory cells derive from their parent B and T cells, and undergo clonal selection following infection, which increases antigen-binding affinity.
MAJOR HISTOCOMPATIBILITY COMPLEX ANTIGENS
The major histocompatibility complex (MHC) is a cell surface molecule that regulates interactions between white blood cells and other cells.
Types of MHC
MHC I is presented on all cells of the body. It contains an epitope that forms the structural binding site for an antigen. MHC I interacts with natural killer (NK) cells and cytotoxic T cells to signal whether a cell is self or non-self, and whether it contains an antigen specific to that T cell.
MHC II is presented mainly on macrophages, dendritic cells, and helper T cells, which are all involved in antigen presentation. It has a longer helical region than MHC I, which allows it to bind to CD4 (helper T cells) during antigen presentation.
MHC III is a secreted enzyme that is neither membrane-bound nor involved in antigen presentation like MHC I and II. It is merely included as an MHC protein because it is encoded by the same set of genes. It is involved in production of complement proteins and inflammatory cytokines.
HLA and Organ Rejection
The damage in organ rejection can be acute or chronic, cell-mediated or antibody -mediated, and often involves diffuse damage of the graft that cause necrosis and infarction ( tissue death from lack of oxygen) to the graft tissue by attacking its vascular components.
surgically-replaced organs are often rejected by the body’s immune system.
ANTIBODY
An antibody is a Y-shaped protein produced by B cells to identify and neutralize antigens in the body.
An antibody (formally called immunoglobulin) is a large Y-shaped glycoprotein produced by B-cells and used by the immune system to identify and neutralize pathogens. Antibodies are produced by B cells, and are either secreted into circulation or remain expressed on the surface of the B cell.
SANDHYA A 191822016