What is immunity?
Immunology is like a colorful puzzle piece in the big picture of biology. It’s like a key that helps us unlock the secrets of living things. Now, if you’re thinking, “What’s all this talk about immunity?” don’t worry! Imagine it as your body’s superpower to protect itself. But before we jump into all the cool details, let’s first understand what immunity really means.
When we talk about immunity, we’re talking about how your body can defend itself against things that could harm it. Think of it as your body’s shield against tiny invaders like germs and harmful stuff. And if your body doesn’t have this shield, it’s more vulnerable – we call that susceptibility. This is where immunology comes in. It’s like a guide that helps us understand and use the body’s defenses.
Here’s a fun fact: the word “immunity” was first used by someone named Burnet. But the real superhero of immunology is Emil Von Behring, who discovered something called antibodies. So, get ready to explore the amazing world of immunology, where we learn how your body’s defenses work and uncover some awesome discoveries!
In our body, there are two patterns or mechanisms for defense. What are these two mechanisms? In the chapter “Human Health and Disease,” we had studied that diseases fall under two categories — the first one is congenital (disease from birth), and the second one is acquired (diseases acquired after birth). Almost the same scenario is seen in the case of immunity. The two defense mechanisms are mentioned below:
Note: In non-specific immunity, the first line of defence and second line of defence are included.
(A) Non-specific defense / innate immunity / natural immunity — Such defense mechanisms are present in our body right from the birh.
(B) Specific defense / acquired immunity / adaptive immunity — Such defense mechanisms are acquired by the body after birth.
Non-specific defense / innate immunity / natural immunity:
- It is inherited by organisms from their parents
- Protect organisms from birth throughout life and prevents entry of microorganisms to reach in blood.
- It lacks specific response to specific invaders: When a pathogen enters into our body, it carries weapons with itself. The weapon of pathogens is called antigen using which it causes destruction inside our body. On the other hand, the body produces a counter weapon against a specific antigen. In other words, for every specific antigen, body produces a specific antibody. In case of non specific defense, there is no distinction of antigens, meaning all antigens are treated as a foreign body and they are not allowed to enter into our body.
- It is a broad spectrum defense mechanism: When there is no specific defense mechanism present in our body or the defense mechanism treats all foreign bodies equally irrespective of their nature, such defense mechanism is called broad spectrum defense mechanism.
- Immunological memory is not generated in this type of immunity: In case of small pox, we have studied that if a person is having small pox once, he or she will not have small pox another time. It means, the weapon (antibody) formed by the body once recognizes the small pox antigen. if it enters the body again, it is destroyed by the body.
- It is the first line of defense or it is an external defense that prevents entry of microorganisms in blood. Physical and chemical barriers (physiological) come under the first line of defence.
- It is made up of four types of barriers
Barriers of Innate Immunity
(a) Physical Barriers
(b) Physiological Barriers
(c) Cellular Barriers
(d) Cytokine barriers
Physical Barriers: (First Line of Defence)
They are actually mechanical barriers, also called as structural/anatomical barrier and are of two types:
(1) Skin (external barrier): Intact skin is the largest defensive organ. If a pathogen has to enter the body, it needs to cross the skin and thus skin acts as a physical barrier for any kind of foreign body.
Pathogens can infiltrate the body not only through the skin but also via the mouth and urogenital tract. However, once a pathogen enters through the mouth or urogenital tract, it must breach the mucus membrane. Consequently, the mucus membrane also serves as a physical barrier against various foreign bodies.
Before delving further, it’s crucial to grasp the concept of membranes and the different types present in our body. Membranes can be broadly categorized into two types – mucosa and serosa. Let’s begin by understanding the definition of a membrane. A membrane can be defined as an epithelial layer that is connected to a thin layer of underlying tissue.
The first type of membrane is known as mucosa. To comprehend its location in the body, consider all the cavities that have an opening connecting them to the external environment. These cavities are internally lined by mucosa. For instance, take our alimentary canal, which extends from the mouth to the anus. Since this canal is connected to the external environment via the mouth and anus, it is internally lined by mucosa. Another example is the respiratory cavity, which is in direct contact with the external environment and is consequently lined by mucosa.
In simpler terms, mucosa lines all cavities that are linked to the external environment through an opening. Examples include the intestinal cavity, oral cavity, and pharynx.
Now, let’s turn our attention to the second type of membrane – serosa. Serosa comes into play in body cavities that are closed and lack any communication with the external environment. Consider the thoracic and abdominal cavities as prime examples. However, serosa always exists in two layers – the parietal layer and the visceral layer – forming two reflective surfaces.
By understanding these membrane types, we gain insight into the body’s intricate protective mechanisms.
Physiological Barriers (sectetory products of the body): (First line of Defence)
(A) Acid of stomach:
The gastric juice produced by the stomach plays a pivotal role in the body’s digestive processes through the release of hydrochloric acid (HCl), which serves as the linchpin for four fundamental functions. Let’s delve into each of these functions, enhancing our engagement with the subject:
- Softening Firm Foods to Enhance Churning Efficiency: The initial purpose of HCl is to tenderize rigid foods. Visualize your stomach as a skilled chef meticulously preparing a culinary masterpiece. In the same way a chef tenderizes meat before cooking, HCl works to soften the robust components of ingested food. This preparation facilitates the churning process, enabling it to work its transformative wonders.
- Establishing an Acidic Environment to Support Enzymatic Activity: Transitioning to the second role, HCl creates an environment characterized by acidity. Envision this environment as a well-orchestrated stage ready for a remarkable performance. Just as actors thrive under the spotlight, stomach enzymes thrive within the acidic backdrop. This acidity is instrumental in effectively breaking down the ingested food, ensuring the seamless progression of the stomach’s digestive spectacle.
- Activation of the Potent Pepsin Enzyme: Our following function involves a metamorphosis – the activation of the pepsin enzyme. Envision this process as akin to a superhero harnessing their powers upon donning their iconic costume. HCl orchestrates the transformation of inactive pepsinogen into its potent incarnation, known as pepsin. This pepsin superhero stands prepared to dismantle proteins and contribute significantly to the digestion process.
- Safeguarding Against Harmful Intruders: The concluding, yet paramount, role of HCl is that of a guardian. Picture your stomach as an impregnable fortress, with HCl acting as the vigilant sentinel at its gates. It serves as an impervious barrier, thwarting the infiltration of detrimental microbes or agents that may attempt to infiltrate along with your food. HCl exercises its vigilance indiscriminately, akin to a watchful bouncer ensuring that only benign guests gain entry to the digestive affair.
And thus, the multi-dimensional character of the stomach’s HCl comes to light. Far from being a mere acidic substance, it functions as a multitasking virtuoso, expertly performing these four indispensable roles.
(B) Bile: If by any means, pathogens are not killed by HCl, then bile will act on them
(C) Cerumen (ear wax)
Cerumen, also referred to as ear wax, assumes a captivating function within our auditory system. Its significance goes beyond mere simplicity, as it serves as a vital defense mechanism. Envision this scenario: as it emerges within the external auditory meatus, it transforms into a proactive protector. Its primary objective? The containment and prevention of pathogens that may attempt to infiltrate the middle ear region, potentially leading to complications. Thus, envision this cerumen as an alert guardian, positioned at the threshold of the middle ear, fully prepared to intercept and counteract any unwelcome intruders.
(D) Lysozyme (antibacterial) — present in the salivary gland, tears, etc. Lysozyme ruptures the cell wall of gram positive bacteria.
(E) Nasal hairs
(F) Urine
(G) Vaginal secretions
(H) Sebum (oily secretions) by skin
Cellular Barriers: (Second Line of Defence)
–– It is internal defence and prevents spread of microorganisms in the body.
-When microorganisms reach the blood.
(1) Leucocytes: There are five distinct types of leukocytes, commonly referred to as White Blood Cells (WBCs). However, only two of these types exhibit phagocytic characteristics – neutrophils and monocytes. Among these two, neutrophils outnumber monocytes. Consequently, when pathogens succeed in breaching the skin and mucous membranes, and subsequently reach the bloodstream, neutrophils assume the forefront in launching an immune response. This strategic role designates neutrophils as the primary phagocytic cells. Additionally, neutrophils are known as Polymorphonuclear Leucocytes (PML) owing to their diverse nuclear lobes, ranging from three to seven, observed within an individual’s blood. Essentially, an individual’s blood may encompass neutrophils with three, five, six, or even seven nuclear lobes.
Monocytes are agranular and largest in size among WBCs. The nucleus of monocyte is horse-shoe shaped. Monocytes are also called as Macrophages (Macro = largest; Phages – Phagocytic in nature).
(2) Macrophages
When monocyte (macrophage), a type of WBC is present in blood, it is called monocyte and it acts as a phagocytic cell (antigen presenting cells). However, the monocyte changes its name according to the location it is present in the body. In other words, macrophages are nothing but monocytes. Following are the names given according to their location:
a. Histiocyte: macrophages present in tissues
b. Kupffer cells: present in liver and functions in phagocytosis.
c. Microglial cells: It is a part of nervous tissue. It is important to mention that the entire nervous tissue is ectodermal in nature except mesoglial cell, which is mesodermal in origin.
d. Osteoclast: present in bones and performs in phagocytosis
e. Chondroclast: present in cartilage and performs in phagovytosis
f. Mesangial cells: present in the kidney and performs in phagocytosis
(3) Natural Killer Cells (NK Cells)
It is a type of lymphocyte (non-phagocytic). They are found in spleen, lymph nodes and red bone marrow (RBW).
Before getting into the details of the mechanism of cell destruction by NK cells, let’s first understand the meaning of self cell and altered self cell (ASC). The meaning of self cell is simple — all normal cells present within our body come under self cell. On the other hand, altered self cells are those normal self cells that have become destorted and harmful for the body. For instance, tumor cells are destorted cells that cause cancer and eventually the death of the organism.
NK cells and Perforins:
If an altered self-cell is detected within the body, the NK cell will approach the cell and establish a connection with it. Once this linkage is formed, the NK cell will release a chemical known as perforin. This chemical, in turn, induces perforations on the plasma membrane of the modified self-cell.
(5) Inflammation:
In a simple language, inflammation means painful swelling. It is due to secretion of histamine (vasodilator) and prostaglandins. Following are the 4 main signs associtated with inflammation:
(a) Pain
(b) Swelling
(c) Redness
(d) Local warmth
Note: any word associated with a suffix “itis” represents a kind of inflammation.
(6) Fever (Pyrexia): It is a sign that something wrong is happening inside the body.
- Inflammatory response may be local or systemic
- Systemic inflammatory response results in fever
- In fever, body temperature increases due to production of pyrogens (temperature generating chemicals like interleukins).
- Hypothalamus is thermostat or temperature regulating center of the body. Prostaglandins stimulate temperature center of hypothalamus to increase body temperature.
- Fever up to 100 degree Fahrenheit is tolerable and it stimulates phagocytosis.
- Fever above 100 is harmful (metabolism is disturbed and there is threat to neurons).
Cytokine Barrier
They are low molecular weight proteins that stimulate or inhicbit differentiation, proliferation and function of immune cells. They are:
(a) Interleukins
(b) TNF (Tumor Necrosis Factor)
(c) Interferons: These are proteins that are released by viral infected cell to protect neighboring cells from viruses. In other words, when a cell is infected by a virus, the infected cell releases interferons that signal neighboring cells to produce antiviral protein (AVP) to protect them from viruses.
It does not protect cells from bacterial or fungal attacks, but only from viruses. You might ask if interferons are specific for viruses, why it cannot be considered under specific immune system. Well, It is true that interferons are specific for viruses but we consider it under non-specific immune system because it protects cells from any kind of viruses and not specific to a kind of virus.
Types of Interferons:
- Alpha interferons — secreted by leucocytes (B and T cells)
- Beta interferons — secreted by fibroblasts
- Gamma interferons — secreted by NK cells