Topic 2: Cells: The Humoral Response and Antibodies

Humoral response overview

Lymphocytes are white blood cells involved in the specific immune response. The humoral response is the response involving B cells and antibodies

B lymphocytes

have antibodies on their surface membrane to bind to complementary antigens. On doing so, they engulf the antigens and display them on their cell surface to become APCs. Once activated can divide into plasma or memory cells. Approximately 10 million different types of B lymphocytes -each with different antibody

Helper T cells are also involved in the humoral response

These cells bind to antigen presenting cells to activate the division of B cells

Plasma cells

Only live for a few days once activated. Produce 2000 antibodies per second while active for specific antigen. Circulate around site of infection

B memory cells

Can live for decades. Provide long term immunological memory. Circulate in blood and tissue fluid (lymph). Do not produce antibodies (involved in secondary response). Can rapidly divide into plasma cells if the body is reinfected by the same pathogen

B lymphocyte activation

1. Antigens in the blood collide with their complementary antibody on the B cell. The B Cell takes in the antigen by endocytosis and then presents on its cell surface membrane

2. When this B cell collides with helper T cell receptors, this activates the cell to go through clonal expansion

3. B cell undergo mitosis to make large numbers of cells, these differentiate into plasma cells or memory B cells (clonal expansion)

4. Plasma cells produce and secrete the specific antibodies which are complementary to the antigen on the pathogen’s surface. These antibodies attach to the antigens on the pathogens and destroy them

5. B memory cells can divide rapidly into plasma cells when reinfects with the same pathogen to make large numbers of antibodies rapidly (circulate in blood and tissue fluid)

Clonal selection

Point at which the B cell with correct antibody to particular antigen is selected for cloning (by being activated by T helper cell)

Clonal expansion

Interleukins (a type of cytokine) produced by activated T helper cells activate the B cells and they divide by mitosis to produce clones

Antibodies

Y-shaped glycoproteins made up of four polypeptide chains

Antibody structure

• Quaternary structure proteins made up of four polypeptide chains

• Each different antibody has a different shaped binding site, which is the variable region. The shape of the antigen-binding site is unique to the shake of a particular antigen

• When they bind form an antigen-antibody complex

• Variable regions have a specific amino acid sequence for each antigen

Antigens do not destroy pathogens directly. They prepare the antigen for destruction:

• mark pathogens for phagocytosis

• Neutralising toxins - antibodies bind to toxins to inactivate them - stops damage

• Agglutination

• Preventing pathogens from binding - antibodies bind to pathogens to stop them from infecting body cells, bind or invade

Agglutination

Antibodies are flexible and can bind to multiple antigens to clump them together.

Makes it easier for phagocytes to locate and destroy pathogens

The primary response produces a lesser concentration of antibodies than a secondary immune response

Initial immediate response to an antigen producing antibodies and memory cells. Slow and infected individual experiences symptoms of the disease. Plasma cells secrete antibodies into the blood plasma each cell can make approximately 2000 antibodies per second, but the cell only survives a few days

The secondary response is when the infection is seen for a second time and is initiated by memory cells

Memory cells encounter same antigen at a later date and divide (and develop) into plasma cells and more memory cells. Response is much faster and pathogens are destroyed before any symptoms appear. Larger response is seen (more antibodies) with a reduced amount of antigen. It’s more rapid so that the pathogen can be destroyed before it fully infected the body and causes disease- long-term immunity.