In immunology, understanding immunity means looking at its different parts, including innate and adaptive responses. Let’s start on a journey through the key players and processes that protect our bodies against germs.

Innate Immunity: The First Line of Defence

Barriers

The body many layers of defence to fight off pathogens.. Mechanical barriers include intact skin and mucous membranes. These membranes line organs like the respiratory and gastrointestinal tracts. They physically obstruct pathogens from entering or penetrating deeper tissues. Complementing these are chemical barriers, including sweat, sebaceous secretions, saliva, and gastric acid, which contain antimicrobial agents like peptides, fatty acids, and enzymes like lysozyme, capable of neutralizing or destroying pathogens. Additionally, the body’s normal microbiota act as microbial barriers. They outcompete pathogens for resources. They also produce antimicrobial substances. This process maintains a healthy balance. Keep in mind that this is just an overview of the immune system components. Because, the Innate immune system will be discussed in more detail in a separate article.

Innate Cells:

Natural Killer (NK) Cells and Antigen Presenting Cells (APCs) are vital components of the immune system. Additionally, NK cells, a type of lymphocyte, possess cytotoxic abilities and target virus-infected or tumour cells by detecting surface marker changes. As a result, they induce cell death in the target through the release of cytotoxic granules. Meanwhile, APCs, including dendritic cells, macrophages, and B cells, bridge innate and adaptive immunity by capturing antigens and presenting them to T cells, initiating adaptive responses. Together, NK cells and APCs coordinate immune responses, ensuring effective defence against pathogens and abnormal cells.

Immune response progresses through three key phases. First is Recognition, where innate cells recognize pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors. Second is Activation, triggering various effector mechanisms to combat pathogens. Finally, Elimination encompasses processes like phagocytosis, extracellular killing, and inflammation to neutralize threats.

Soluble Factors:

Interferons, complement proteins, acute phase proteins, and enzymes are vital components of innate immunity. Interferons inhibit viral replication and boost antiviral defences, while complement proteins enhance immune responses by opsonizing pathogens, inducing inflammation, and lysing microbial membranes. Acute phase proteins aid in pathogen clearance during inflammation, and antimicrobial enzymes like lysozyme and lactoferrin inhibit pathogen growth by targeting bacterial cell walls and essential nutrients. These elements collectively fortify the body’s innate immune defences against microbial threats.

Innate Vs Adaptive Immunity

Innate Immunity	Adaptive Immunity
Born with it	Stimulated by pathogens
First line – acts immediately	Second line – takes time
Less specific	Specific
No memory	Memory
Only acts on foreign substances	Can act on self tissues
Recognizes broad molecules on pathogens by a small set of receptors (Pattern Recognition Receptors)	Recognizes specific molecules (Antigens) on pathogens by a vast array of receptors (TCR, BCR)
Macrophages	Antigen Presenting Cells
Granulocytes	T-Cells
Natural Killer (NK) cells	B-Cells
Other chemicals (HCl, Lysozyme)	Antibodies
Complement	Complement

Adaptive Immunity

Immunity operates as a complex interplay of cells, proteins, and processes, from the rapid innate response to the tailored defences of adaptive immunity.

Humoral vs Cell Mediated Immunity

Humoral immunity, involving B cells, yields antibodies secreted into blood and mucosal fluids, combating pathogens. In contrast, cell-mediated immunity, reliant on MHC molecules, engages a spectrum of lymphocytes including T cells, which discern processed antigens via MHC molecules. Through distinctive mechanisms, both branches foster adoptive immunity, with B cells activating upon antigen binding and T cells requiring co-stimulation. Witness their orchestrated responses, from clonal expansion to memory cell formation, against extracellular and intracellular threats.
Both Humoral Immunity and Cell Mediated Immunity will be discussed in great detail in separate articles.

Humoral	Cell Mediated
Done by B cells and antibodies	Done by: Antigen specific — CD4+, CD8+ T cells Non-specific — Macrophages, neutrophils, NK cells
B cell receptors can identify unprocessed antigens	T cells recognize processed antigens presented by APC on MHC molecules
Receptors: Membrane-bound antibodies	Receptors: T cell receptors
Activation signals 1st: Binding of antigen to BCR, 2nd: From activated T cells or microbes/accessory cells	Activation signals 1st: Binding of TCR to peptide antigen on MHC, 2nd: From co-stimulators
After activation — Clonal expansion, somatic hypermutation, affinity maturation, formation of memory cells	After activation — Formation of memory cells with the help of T helper cells
Special features: Specificity, diversity, memory, clonal expansion, specialization, self-limiting response	—
Kill extracellular pathogens	Kill intracellular pathogens and tumour cells

T-Cells:

T-cells are a key component of adaptive immunity. They undergo maturation in the thymus. They are responsible for orchestrating highly specific immune responses against pathogens. There are several subtypes of T-cells, each with distinct functions:

• Helper T-cells (CD4+): These cells play a central role in coordinating immune responses by secreting cytokines that activate other immune cells. Helper T-cells are crucial for the activation of B-cells, cytotoxic T-cells, and macrophages, thereby enhancing the overall immune response.
• Cytotoxic T-cells (CD8+): Cytotoxic T-cells are specialized in recognizing and eliminating cells infected with intracellular pathogens, like viruses. They induce apoptosis in infected cells by releasing cytotoxic granules containing perforin and granzymes, effectively eliminating the source of infection.
• Regulatory T-cells (T-regs): Regulatory T-cells are involved in maintaining immune tolerance and preventing autoimmune responses. They suppress excessive immune activation and inflammation, helping to prevent immune-mediated tissue damage.

B-Cells and Antibodies:

B-cells are lymphocytes responsible for humoral immunity, which involves the production of antibodies that target specific antigens. Upon encountering an antigen, B-cells undergo activation and differentiation into plasma cells, which secrete large quantities of antibodies tailored to bind with high specificity to the antigen. Antibodies neutralize pathogens by blocking their entry into host cells, promoting their phagocytosis by immune cells, and activating the complement system to enhance their clearance.

Complement System

The complement system, comprising a series of soluble proteins, serves as a bridge between innate and adaptive immunity. While traditionally considered part of the innate immune response, the complement system also plays a crucial role in shaping adaptive immune responses. Complement proteins enhance the phagocytic activity of macrophages and neutrophils, promote inflammation, and aid the clearance of immune complexes. Additionally, complement activation can augment adaptive immune responses by promoting the activation and maturation of B-cells and assisting in the opsonization of pathogens for recognition by phagocytes. The complement system and the complement cascade will be discussed in a separate article.