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Primary defence system:

-Skin composed of dead cells containing an indigestible protein keratin. The skin also produces sebum, which lowers pH inhibiting pathogenic growth.

-Lysozymes in tears, saliva, sweat are antibacterial agents and can kill pathogens.

-Respiratory tract has mucus which traps pathogens, whilst cilia waft mucus towards the stomach, protecting gas exchange surfaces that are important.

-Bacteria are destroyed in the stomach by the low pH HCl (hydrochloric acid).

Antigens and recognition:

Antigen: Surface protein/glycoprotein/glycolipid which gives cellular recognition for bacteria/viruses/cells. Stimulate antibody production.

Inflammatory response:

-Histamine is released into the wound by white blood cells (lymphoctyes), increasing vasodilation events as well as vascular permeability.

-Vasodilation (much like the response against heat) and the area becomes warm and red.

-Increased vascular permeability means that more tissue fluid (containing plasma proteins and antibodies) can move into the tissues, causing swelling.

Phagocytosis (non-specific):

-Phagocytes are examples of immune cells that destroy pathogens. The most notable example here is the macrophage ('large eater')

-Phagocyte is attracted to pathogen by chemotaxis (pathogen gives off chemicals, phagocyte attracted to pathogen along chemical gradient.

-Antibodies cause opsonisation of bacteria/pathogen (fully coated) increasing the binding efficiency between the bacteria and phagocyte.

-Phagocyte forms psuedopodia around pathogen (extending sleeve of cytoplasm) encasing pathogen in phagosome.

-Vesicles containing hydrolytic enzymes (lysosomes) move towards the newly formed phagosome and fuse with it, killing the pathogen with digestive enzymes.

-Phagocyte may become APC once this process is complete.

Antibody function:

-Antibodies are immune proteins made by plasma cells. Their function is to attach by complementary protein interactions to their antigen (each antibody is specific to an antigen).

-Antibodies help mark pathogens as foreign invaders by attaching to their surface antigens.

-Antibodies are responsible for agglutination (the clumping together of multiple pathogenic entities) to allow for rapid en masse degradation using phagocytes.

-Antibodies are also responsible for carrying out opsonisation (coating the pathogen fully).

Antibody structure vs function:

-Variable region: Specific tertiary structure adopted in order to bind antigen with high specificity and complementarity. Similar to enzyme active site idea.

-Disulphide bridge: Strong bonds holding the heavy chains together. Disulphide bridges typically arise from cysteine residues, which contain sulphur.

-Fc region: Conserved across antibodies, the Fc region adopts a constant tertiary structure. This allows immune cells (lymphocytes) to bind to the Fc region of any antibody.

The Humoral Response:

-Pathogen ingested by a macrophage and antigen is displayed on the surface (antigen presenting cell-APC).

-Th cells (T-helper) bind to antigen by complementary protein-protein interactions and release cytokines to activate b-cells. This is known as clonal selection.

-Rapid clonal selection and expansion (production of antibodies) occurs to strengthen the response.

-Th cells also divide by mitosis to form clones, whilst plasma cells are also formed, which produce large amounts of the specific antibody.

-Immunological memory is stored within memory cells, which can rapidly differentiate upon 2nd encounter with disease.

The Cell-Mediated Response:

-Cell becomes an APC through viral infection/cell takeover. MHC proteins are expressed normally, but interact with the non-self antigen.

-Specific T-lymphocyte recognises MHC abnormality and differentiate into different types of cell.

-Cytotoxic T-cells destroy pathogens and infected cells by secreting damaging enzymes into them (perforins + granzymes).

-Helper T-cells release cytokines to stimulate other immune t-lymphocytes. These are destroyed by HIV.

-Memory T-cells are formed to retain immunological memory.

HIV: Human Immunodeficiency Virus

AIDS: Acquired Immunodeficiency Syndrome

Antibody structure:

Variable region

Light chain

Disulphide Bridge

Heavy chain

Fc region (constant)

Vaccination:

-The word vaccine comes from the Latin vacca, for cow. This is because the earliest forms of inoculation and vaccination involved using cowpox to generate immunity against smallpox.

-Vaccines contain weak or inactive forms of a given pathogen. When exposed to this pathogen, our bodies carry out the cell mediated and humoral response in order to generate immunological memory.

-The next time someone encounters the antigen from the vaccine, they will be able to rapidly generate more antibodies as well as other immune cells to help clear the infection faster.

-Modern vaccines e.g the Pfizer vaccine is different in the sense that is uses the mRNA instead of a protein component. This means our bodies endogenously produce the spike proteins found on the coronaviruses lipid coat.

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