Question

Describe as detailed as possible how an individual mount an immune response to infectious bacteria, explaining all the steps from the bacteria break the outer barriers to the individual obtains a long-lasting immunity.

Answer 1

1. Bacteria that enter the body break through skin or any other openings (nose, eyes, ears etc.,). When they enter the body of the host they are recognized as foreign by mast cells and dendritic cells. The pathogens contain Pathogen Associated Molecular Patterns (PAMPs) which are conserved molecules that are common to pathogens like peptidoglycan molecules or the protein flagellin that are a part of the bacterial cells. They are recognized by the hosts body by Pattern Recognition Receptors like Toll Like Receptors.

2. The receptors give rise to signal transduction activating the primary immune response which is inflammation. The mast cells release histamine (dilates the blood vessels and allows more blood flow) heparin (acts as anti-coagulant) and cytokines (which act as signaling molecules and bring in macrophages and neutrophils). Macrophages engulf the pathogens and neutrophils increase the inflammatory response by secreting cytokines and and carry out phagocytosis and by releasing NETs (Neutrophil Extracellular Traps) which are neutrophil DNA that bind to the pathogen molecules on their cell walls and destroy them.

3. Dendritic cells are Antigen Presenting Cells (APCs) which engulf the antigens of the pathogens and break them into smaller molecules called epitopes and present them to Major Histocompatibility Complex (MHC) 2 and the move to the lymph nodes.

4. The dendritic cells present the antigen to the T-cells and leading to stabilization of the connection by binding of CD4 or CD8 (glycoproteins on the membranes of the T- cells) cells. The T-cells now divide rapidly and reach the site of infection and get further activated by the release of cytokines by the dendritic cells mast cells etc., B cells are also activated by binding to the antigen presented by T-helper cells or to free antigens. to their receptors called B cell receptors which contain an antibody (membrane bound) and proteins CD79a and CD79b. The antigen is broken down inside the B cells and an epitope is displayed on its surface attached to MHC class 2 protein. Now the antigen binds to the T helper cell which releases cytokines which help in the activation of B cells. The B cells differentiate into Plasma cells (which produce antibodies) and and Memory B cells (which can store memory of the antibodies for future immunity against the same bacteria). The antibodies that bind to the antigens of the bacteria makes them susceptible for destruction by macrophages and neutrophils.

Thus the host mounts an immune response to destroy the bacteria and also provides a long lasting immunityin case of future infection by the same bacteria.