Immune System and Historical Perspectives

Edward Jenner discovered the first vaccine using cowpox to prevent smallpox.
Louis Pasteur expanded on Jenner's work, developing vaccines for rabies and anthrax.
Elie Metchnikoff identified phagocytosis, the process by which certain cells engulf and destroy foreign invaders.

Innate immunity is the body's initial, non-specific defense against pathogens.
Involves physical barriers like skin, mucous membranes, and chemical defenses like stomach acid and enzymes.
Adaptive immunity is a specific and targeted response to particular pathogens.
Requires time to develop but provides long-lasting protection, including memory responses.

Hematopoiesis is the process of blood cell formation, occurring in the bone marrow.
Lymphoid cells originate from the bone marrow and include T lymphocytes, B lymphocytes, and NK cells, all involved in adaptive immunity.
Macrophages are phagocytic cells that engulf and destroy pathogens, acting as antigen-presenting cells in innate and adaptive immunity.
Granulocytes are another type of phagocytic cell, including neutrophils, eosinophils, and basophils, each with specific roles in inflammation and pathogen defense.

Primary lymphoid organs are where immune cells develop and mature, including the bone marrow (for B cells) and thymus (for T cells).
Secondary lymphoid organs are where immune responses are initiated and take place, including lymph nodes, spleen, and mucosal tissues.
These organs act as filters for pathogens and sites where lymphocytes encounter antigens and mount immune responses.

Antigens are substances that trigger immune responses, including proteins, carbohydrates, and lipids.
Epitopes are specific regions on antigens that are recognized by antibodies or T cell receptors.
Haptens are small molecules that cannot trigger an immune response on their own but can do so if they bind to larger carrier molecules.
Adjuvants enhance the immune response to antigens, often used in vaccines.
Antibodies are proteins produced by B lymphocytes that specifically bind to antigens, neutralizing them and marking them for destruction.
Different classes of antibodies (IgG, IgM, IgA, IgE, IgD) have distinct structures and functions in the immune response.

B cell activation requires antigen binding to the B cell receptor, followed by signaling events that lead to proliferation and differentiation into plasma cells, which produce antibodies.
T cell activation requires antigen presentation by antigen-presenting cells (APCs), such as macrophages and dendritic cells.
T cell activation involves the interaction of the T cell receptor with MHC-peptide complexes and co-stimulatory molecules on the APC.
Cytotoxic T lymphocytes (CTLs) directly kill infected cells, whereas helper T cells activate other immune cells to carry out the immune response.
Cytokines are signaling molecules that regulate immune cell function, including activation, differentiation, and proliferation.

The complement system is a group of proteins that circulate in the blood and become activated in response to pathogens.
Its action includes direct lysis of pathogens, opsonization (enhancing phagocytosis), and recruitment of inflammatory cells.
Three activation pathways are involved: the classical pathway, the alternative pathway, and the lectin pathway.

Major Histocompatibility Complex (MHC) is a group of genes responsible for producing cell surface proteins that present antigens to T cells.
MHC Class I molecules present antigens derived from intracellular pathogens to cytotoxic CD8+ T cells.
MHC Class II molecules present antigens derived from extracellular pathogens to helper CD4+ T cells.
T cell receptors (TCRs) are unique, antigen-specific receptors on T cells that interact with MHC-peptide complexes.

Hypersensitivity reactions are exaggerated immune responses to antigens, causing tissue damage.
Type I hypersensitivity is immediate and IgE-mediated, as seen in allergic reactions.
Type II hypersensitivity involves antibodies binding to cell surface antigens, leading to cell destruction.
Type III hypersensitivity involves immune complexes forming in tissues, causing inflammation.
Type IV hypersensitivity(delayed-type hypersensitivity) is cell-mediated and mediated by T cells, as seen in contact dermatitis.
HLA antigens are highly polymorphic, meaning they vary greatly between individuals, which influences immune responses and tissue compatibility.

Viral Infections:
Specific immune responses to viral infections depend on the virus.
For HIV infection, the virus targets the immune system, particularly CD4+ T cells, leading to immunodeficiency.
Bacterial Infections:
The body fights bacterial infections through both innate and adaptive mechanisms.
Some bacteria, like Mycobacterium tuberculosis, have developed mechanisms to evade the immune system.
Protozoan Infections:
Malaria is a parasitic infection caused by the protozoan Plasmodium.
The parasite evades the immune system by changing its surface antigens and infecting red blood cells.

Precipitation reactions involve antigen-antibody interactions forming visible precipitates.
Agglutination reactions involve antigen-antibody interactions causing clumping of cells or particles.
ELISA (enzyme-linked immunosorbent assay) is a widely used technique for detecting and quantifying antigens or antibodies.
RIA (radioimmunoassay) is a similar technique but uses radioactive isotopes to detect antigen or antibody.
Western blotting separates proteins by electrophoresis and uses antibodies to detect specific proteins.
FACS (fluorescence-activated cell sorting) uses fluorescent antibodies to identify and sort cells based on their specific surface markers.

Autoimmune disorders occur when the immune system attacks the body's own tissues.
Mechanisms include genetic predisposition, environmental factors, and a breakdown in self-tolerance.
Vaccines work by introducing a weakened or inactive form of a pathogen to stimulate an immune response, preparing the body for future encounters with the real pathogen.
Types of vaccines include:
- Active vaccines - introduce the weakened or inactive pathogen or its components.
- Passive vaccines - introduce pre-made antibodies to provide immediate but short-lived protection.
- Purified vaccines - contain specific proteins, such as toxoids, from the pathogen.
- Recombinant vaccines - are produced using genetically engineered organisms to express specific viral or bacterial antigens.
- Subunit vaccines - contain only specific parts of the pathogen, such as capsular polysaccharides.
- DNA vaccines - use plasmids containing genes encoding specific pathogen antigens to induce an immune response.
Recent examples include the COVID-19 vaccines Covaxin (inactivated virus vaccine) and Covishield (viral vector vaccine).