Immunity - Session 2

Antibodies are specialized Y-shaped proteins produced by the immune system. Their primary function is to identify and neutralize foreign substances such as bacteria, viruses and toxins, which are collectively known as antigens. They can coat pathogens, making them easier for phagocytes to recognize and engulf. Antibodies can bind multiple antigens simultaneously, causing them to clump together. This makes it easier for the immune system to target and eliminate them. Can you tell the difference between a pathogen and an antigen?.

Lymphocytes are a type of white blood cell that plays a pivotal role in the immune system. They are primarily responsible for identifying, targeting and remembering specific pathogens to protect the body against infections. Lymphocytes are broadly categorized into three main types. These are B cells, T cells and natural killer cells. Each type has distinct functions and characteristics.

B cells mature in the bone marrow. They express unique receptors known as B cell receptors on their surfaces, which can bind to specific antigens. Once activated by the antigen, B cells can differentiate into plasma cells that produce antibodies. These antibodies bind to the antigen, marking it for destruction or neutralization. Some activated B cells become Memory B cells, which remain in the body for long term. They can quickly respond to future infections by the same pathogen.

T cells mature in the thymus. They express T cell receptors on their surfaces. T cell receptors recognize antigens presented by other cells. There are different types of T cells. These are Helper T cells, Cytotoxic T cells and Regulatory T cells.

When a pathogen enters the body, it brings along its antigens. These antigens are unique markers that the immune system can recognize as foreign. These antigens are detected and captured by the antigen presenting cells. There are different types of antigen presenting cells. These are dendritic cells, macrophages and B cells.

Dendritic cells are found in tissues that are in-contact with the environment, like the skin and mucous membranes. When they encounter a pathogen, they engulf it. Macrophages are found throughout the body. They also engulf pathogens. B cells can directly bind specific antigens using their B cell receptors.

Inside the antigen presenting cell, the pathogen is broken up into smaller pieces. These pieces are called antigen peptides. Antigen peptides are then loaded onto special molecules called major histocompatibility complex molecules. There are two types of major histocompatibility complex molecules.

After capturing and processing the antigens, dendritic cells travel to the lymph nodes, which are like hubs where immune cells gather and communicate. Naive T cells are T cells that have not yet encountered an antigen. In the lymph nodes, dendritic cells produce the antigen MHC on their surface to naive T cells. For a T cell to become fully activated, it needs to recognize the antigen MHC complex and receive additional signals from co-stimulatory molecules on the antigen presenting cell.

Helper T cells recognize antigens presented by MHC class II. Once activated, they proliferate and differentiate into different types that help other immune cells. T killer cells recognize antigens presented by MHC class I. Once activated, they proliferate and become cytotoxic T-lymphocytes that can kill infected cells.

B cells also need help from T cells to become fully activated. B cells bind specific antigens through their receptors, internalize them, and make them available on MHC class II molecules. Activated Helper T cells recognize the antigen-MHC class II complex on B cells. They provide additional signals and secrete cytokines that help activate B cells.

Some B cells become plasma cells, which are antibody factories. They produce antibodies specific to the antigen.Some B cells become Memory Cells. Memory cells stay in the body for long term and respond quickly if the same antigen is encountered again.

Now the immune system is ready to eliminate the pathogen. Antibodies produced by plasma cells can neutralize pathogens by binding to them and preventing them from entering cells.Antibodies can also opsonize pathogens, making them easier for phagocytes to engulf. Cytotoxic T cells kill infected cells by inducing apoptosis. Apoptosis is the programmed cell death. The combined actions of antibodies, cytotoxic T cells and phagocytes work to clear the pathogen from the body.

The secondary immune response is the reaction of the immune system when it encounters an antigen it has previously been exposed to. This response is faster, stronger and more effective than the primary immune response. When the body is re-exposed to an antigen, the immune system quickly recognizes it. We know that the Memory B cells were generated during the primary immune response. Memory B cells have receptors specific to the previously encountered antigen.

Upon re-exposure, the Memory B cells rapidly differentiate into plasma cells. Plasma cells produce large quantities of high affinity antibodies. The secondary response mainly produces IgG antibodies, which are more effective than IgM antibodies produced during the primary response. These antibodies bind more strongly to the antigen because they have undergone a process of fine-tuning during the primary response.

Memory T cells also remember the antigen and react quickly. Memory T cells quickly multiply and turn into Helper T cells and cytotoxic T cells. Helper T cells release signals that help other immune cells, like macrophages and B cells, to fight the pathogen. Cytotoxic T cells attack and destroy infected cells that have the antigen.